The Interosseous Palmaris I of Henle

Samuel Strong Dunlap, Ph.D. Reston, Virginia and M.A. Aziz, Ph.D. Howard University College of Medicine Washington, D.C.

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: hand muscles, ontogeny, Primates, human trisomy, atavism.

© copyright 2013 Samuel Strong Dunlap, Ph.D.

“What , then, is the interosseus primus volaris of Henle? Why, it is evidently just what Henle named it; it is the equivalent in man of the slip of the flexor brevis profundus which arises from the first metacarpal and inserts into the proximal phalanx of the thumb...” McMurrich, 1903 pp. 492-493.

INTRODUCTION

In our dissections of 30 human hands from karyotypically normal adults in a cadaver population from the gross anatomy lab at Howard University Medical School, we have been impressed by the variability seen in the intrinsic hand muscle, interosseous palmaris I of Henle. Our initial interest in this particular deep muscle in the “Mount of VENUS” came about from its evolutionary importance as a second flexor brevis profundus in tetrapods ( Miner, 1924; Bunnel, 1942, 1956; Lewis, l989 ), as an ontogenetic precursor of most of the thenar compartment muscles (Cihak 1972) and, by its retention and variability in human trisomies, hominoids and some Anthropoidea ( Dunlap & Aziz, 1985; Dunlap et al., 1985; Dunlap et al., 1986 ; Dunlap and Aziz, 2012). Furthermore, the interosseous palmaris I of Henle has often been confused for the deep head of the flexor pollicis brevis, the oblique head of the adductor pollicis, or altogether ignored. As we noted in a related paper (Dunlap and Aziz, 2012) considerable confusion exists in the literature on this muscle. In order to dispel this confusion, we report here on our dissections of 30 adult human hands focusing specifically on the interosseous palmaris I of Henle and the adjacent muscle slips with which it has been confused in the literature. Excellent illustrations in the literature may be found in Robinson (1931, p. 466, fig.392), Romanes (1966, p. 116, fig.80) and, Gardner et al. (1975, p. 144, fig. 16-5) and excellent color photographs of dissections may be seen in Zancolli and Cozzi (1992, pp. 286-287, plate 4-4; pp. 306-307, plates 4-13A and 4-13B and pp. 552-553, plates 6-35A and 6-35B). Other names for the interosseous palmaris I of Henle include: interosseus primus volaris of Henle (Wood Jones 1942; St John Brooks, 1886b; McMurrich, 1903); first palmar interosseous (Romanes, 1966; Breathnach, 1965; Fahrer, 1981; Piersol, 1903); interosseous palmaris I Henlei (Cihak, 1972); interosseus palmaris primus (Goss, 1973; St John Brooks, 1886a). We prefer to use the name interosseous palmaris I of Henle because it emphatically identifies it as a first palmar interosseous of the first digit, the thumb, which is often referred to in the literature by Roman numeral I. Additionally, Jacob Henle’s name should be attached to this muscle for the probable earliest correct identification of the muscle. Henle, born the same year (1809) as Charles Darwin, was also an important transitional figure in the emergence of microscopic anatomy in the early 19th century (Lesky, 1976). Despite variable expression in the intrinsic hand muscles, there is overwhelming agreement regarding the individual status of the abductor pollicis brevis and the opponens pollicis. However, the flexor pollicis brevis has been the topic of contentious debate which began in the 1880’s and remaining unresolved to the present ( Cunningham, 1887; Brooks, 1886b, 1887; Flemming, 1887a, 1887b; McMurrich, 1903; Wood Jones, 1942; Straus, 1946a; Abramowitz, 1955; Day and Napier, 1961, 1963; Lewis, 1965, 1989; Cihak, 1972, 1977; Van Sint Jan and Rooze, 1994 and Dunlap and Aziz, 2012). According to McMurrich (1903) the flexor pollicis brevis is “... a compound structure...” (p. 491 ). Regarding its superficial head the consensus is that it is a singular anatomical / functional entity. It originates from the radial part of the distal flexor retinaculum as well as the tubercle of the trapezium, and inserts on the radial aspect of the base of the proximal pollical phalanx. The insertion tendon encloses the radial sesamoid of the first metacarpo-phalangeal joint. It is located lateral to the tendon of the flexor pollicis longus; its insertion (and its associated radial sesamoid) is lateral to the location of the long pollical flexor. It is the deep head of Cruveilhier which has and continues to confound easy determination. Since the 1880’s three distinct slips have been variously recognized as a deep head of flexor pollicis brevis. We here cite Wood Jones (1942, p. 259) clear exposition of these slips: 1. “A deeper portion that arises from the trapezoid and the os magnum, which is deep to the tendon of the flexor pollicis longus and is also inserted into the radial sesamoid and the base of the first phalanx. This portion constitutes the deep head of the flexor brevis of Cruveilhier...” 2. “A small portion that arises from the ulnar aspect of the base of the metacarpal of the thumb and runs along the ulnar side of the tendon of the flexor pollicis longus to be inserted into the ulnar sesamoid and the base of the first phalanx in common with the oblique fibers of the adductor pollicis. ...regarded as the first of the palmar interosseous series - the interosseous primus volaris of Henle.” 3. “A portion that arises from the os magnum and bases of the second and third metacarpal bones and is inserted into the ulnar sesamoid.” “ ... it is now practically universally recognized as a part of the adductor.” The fractionation of the flexor pollicis brevis is advocated on the basis of : attachments, morphology and topography. However, the strongest argument for the division of this muscle into two venters is nerve supply. Whereas, the superficial head is innervated by the recurrent branch of the median nerve, the deep head of Cruveilhier receives branches from the deep ulnar nerve (Brooks, 1886a, 1886b; McMurrich, 1903; Day and Napier, 1961, 1963). Without denying the possible importance of the muscles diverse nerve supply, the ontogenetic and phylogenetic import of this information continues to be debated (Cunningham, 1882; Haines, 1935; Straus, 1946b; Piatt, 1939. The last major studies on the status of the various components of the deep head of Cruveilhier were conducted by Day and Napier (1961, 1963); Lewis (1989) and Van Sint Jan and Rooze (1994); and the last major reviews of interosseous palmaris I of Henle are by Abramowitz (1955) ; Lewis (1965, 1989) and Morrison and Hill (2011). New insights regarding the ontogeny and phylogeny of primate myology of the hand are provided by Cihak (1972, 1977) . Our research seeks to re-observe the incidence and morphology of the interosseous palmaris I of Henle , the morphology of the deep head of Cruveilhier and the contrahens I . We then discuss the phylogenetic and functional implications of these findings. Having examined most of the relevant literature stretching back into the 1700’s, and finding considerable conflicting positions, we have decided as a point of departure to concentrate our introductory discussion around Wood Jones book “The Hand” ( 1942 ) and Day and Napier’s 1961 and 1963 papers in the Journal of Anatomy and Folia Primatologica respectively. In his Chapter XXII , Wood Jones states that left/right symmetry in the intrinsic muscles is rare - and this is certainly our experience. Moreover, the right side is more developed than the left. His masterful descriptions of the flexor pollicis brevis are exactly as we envision them with the advantage of Cihak’s embryological insights to go along with our own primate and human dissection ( all cited above) . At the bottom of p. 259 Wood Jones states in part that the difficulty of definitions is further complicated by the various “...slips [ which ] are themselves liable to considerable variation.” Interestingly , Wood Jones is neither confused nor deterred by the variations and , from our vantage point, basically gets it right. This view is of course enhanced and enlightened by more recent discovery of Cihak’s embryological investigations, our trisomy dissections and our primate and normal human dissections. We find the interosseous palmaris I of Henle quite variable compared to the other slips: flexor pollicis brevis superficial head, the deep head of Cruveilhier, and the adductor pollicis oblique head (hereinafter referred to as contrahens I). Turning to Day and Napier’ s 1961 paper “The two heads of flexor pollicis brevis”, the most remarkable observation is their initial inclusion of the interosseous palmaris I of Henle in the opening paragraphs , especially an accurate if somewhat diagrammatic illustration of the muscle with all relevant surrounding “controversial” slips. They then proceed to completely ignore this embryologically, phylogenetically and probably functionally/clinically important muscle in the remainder of their paper. In their 1963 paper the interosseous palmaris I of Henle is shown in Figure 1, but is not listed among two subsequent figures or a table. Their Figure 2 (p.126) includes diagrammatic depictions of various configurations of flexor pollicis brevis and adductor pollicis insertions of several Primates , but there is no indication of the presence of interosseous palmaris I of Henle. Their Figure 3 of the thenar muscles of Pongo pygmaeus also does not include interosseous palmaris I of Henle. MATERIALS and METHODS We have completed thenar compartment dissections on 30 adult human hands, fourteen paired and two lefts from different adults, all presumably karyotypically normal, obtained by the Howard University College of Medicine, Anatomy Department’s gross anatomy laboratory willed-body program. Limited and superficial dissection was performed by medical, dental and physical therapy students . We selected only those hands with undisturbed thenar musculature and only superficial investigations of the forelimb or forearm. Our dissections were focused on deep thenar muscular presence and morphology. We paid special attention to origins, insertions, locations, duplications and tendinous attachments and expansions. Since our dissections were focused on the interosseous palmaris I of Henle, we also focused particularly on the surrounding muscles: opponens pollicis, flexor pollicis brevis superficial head, the deep head of Cruveilhier, the contrahens I, the first dorsal interosseous, and the tendon of flexor pollicis longus. This approach was promoted by the paucity of carefully reported dissections of interosseous palmaris I of Henle along with careful consideration of all the adjacent muscles as well as by the important human and primate embryological investigations of Cihak (1972). Embryologically, considerations in human muscle phylogeny have often been ignored due largely in part to the paucity of ontological studies. However, as we have pointed out (Dunlap et al., 1985; Dunlap and Aziz, 2012) and others (Waddington, 1957, 1975; deBeer, 1958; Simpson, 1961; Dawid, 1982; Wake et al., 1990; Smith, 1993; Brower et al., 1997 ) , embryological considerations should play a major role in establishing character origins, variations and homologies in individuals, populations and their phylogenetic background. As is well known, Darwin (1859) includes embryological evidence as very important when considering the phylogenetic relations between organisims. Darwin’s recognition of the importance of embryological observations was clear to him from his extensive dissections and subsequent systematic work on barnacles (Darwin, 1851). All dissections were carried out using an illuminated three-power magnification lamp and occasionally a binocular dissection microscope following protocols from Cunningham’s Manuel of Practical Anatomy (Romanes, 1966). We often used a dorsal approach to expose the interosseous palmaris I of Henle because the muscle can be difficult to identify from the palmar view by being frequently closely apposed to the deep head of Cruveilhier and /or contrahens I. This apparently was a preferred approach employed by Abramowitz (1955) , Lewis (1965) , Cunningham (1887) and others. We recorded our observations in notes, many colored pencil drawings and photographs. We tabulated our observations in Table 1 below as the presence or absence of six character states for interosseous palmaris I of Henle, the presence of contrahens I and/or the deep head of Cruveilhier inserting across the distal volar surface of metacarpal I and whether the flexor pollicis longus tendon was doubled. Due to our small sample size, more sophisticated statistical manipulations were not applied. RESULTS As noted above we have found considerable variation in the morphology of the interosseous palmaris I of Henle. Our preliminary conclusion is that these variations are an excellent example of the retention of a primitive muscle (in an otherwise derived surrounding structure , the human hand) from which considerable selective pressure has been removed. Nevertheless, the retention of this muscle in all of our 30 hands dissected, accounting for all of their variations, manifest the importance of this muscle as an ontogenetic precursor of most of the muscles of the “Mount of VENUS” and as sequelae of the developmental process as observed by Cihak (1972). In our Table 1 and the accompanying illustrations we describe our findings, listing the presence (star) or absence (blank) of those character states that we deemed important to elucidate certain aspects of the interosseous palmaris I of Henle and the adjacent musculature with which it is often confused. Initially, we note that the interosseous palmaris I of Henle is present in all of 30 hands dissected. In case HU707, a left hand, the muscle was well developed but very difficult to separate from the radialward margin of the contrahens I throughout most of its length. This was the only hand in which the muscle was not clearly separate. Nevertheless, substantial muscle fibers originated from the ulnarward base of the metacarpal I shaft. A large space along the volar surface of the shaft was filled with very loose connective tissue, fat and a doubled princeps pollicis artery.

Table 1

Notes: muscle is almost doubled; some deep fibers of 1st dorsal interosseous originate from sheet. insertion tendon doubled; muscle easily divided. doubled; ulnarward head from deep head of Cruveilhier. 2nd tendon very tiny; proximally disappears just above carpal tunnel. 2nd tendon almost as large as first. largest seen to date. smallest interosseous palmaris I of Henle seen to date. the medial sheet like is proximally tendinous. muscle is even more diminutive than “g” above. 2nd tendon tiny and distal into phalanges. doubled, radial one is sheet like. second tendon very tiny. origin fibers from shaft. The muscle was doubled or multiple in nine hands. Figure 1 (HU 724) shows a doubled muscle in the right hand of a pair. The ulnarward muscle is clearly separated from the radialward, sheet-like and tendinous second portion by the princeps pollicis artery, The sheet-like portion runs the entire length of the metacarpal I shaft. In the right (Figure 2, HU 710) and left hands (HU 711) of a female the muscles are both doubled. Also on the right, separate inserting tendons come from muscle slips which although closely apposed could be quite easily divided for most of their length. On the left two, clearly separate muscle slips represent the interosseous palmaris I of Henle: the medial slip originates from the deeper origins of the deep head of Cruveilhier , and both slips insert deep to the contrahens I. The flexor pollicis longus tendon is doubled in this left hand; its tendons both about the same size. In Figure 3a and b (HU 730) the interosseous palmaris I of Henle is tripled. The tendinous portion is separated from the doubled muscle by: the princeps pollicis artery, fat, and loose connective tissue. The most ulnarward muscle slip is not seen in the ventral view. Incidental findings from this hand (see Figure 3a) include a small remnant of contrahens V. Occasionally in our dissections we see a thickened connective tissue layer superficially located over the ulnarward interossei, medial to the median raphe and superficial to the ulnar profundus nerve branches. Some investigators argue this connective tissue is a remnant of contrahens IIII and/or V (Flower and Murie, 1867; Cihak, 1967, 1972). In our case ( Figure 3a, a R palmar view) we found a small, thin sheet of muscle tissue and connective tissue oriented in the direction of a contrahens V. Many lower mammals and some Primates have a full complement of this contrahentes layer to include contrahens I, II, IIII and V .

Figure 1 HU724 Palmar View

Figure 2 HU710 Palmar View

Figure 3a

(Straus, 1946a; Stein, 1981; Dunlap et al., 1985; Dunlap, 2011).

Figures 1 - 6. All adult hands.

a. interosseous palmaris I of Henle

b. opponens pollicis insertion

c. contrahens I

d. princeps pollicis artery

e. median raphe

f. first dorsal interosseous

g. contrahens V

h. deep head of Cruveilhier

Fourteen of the muscles exhibited a sheet-like morphology: bilaterally in five individuals and in the left and right hands of four separate individuals. In the bilateral case (HU 702, Figure 4 and HU 703) the muscle was very flat and thin with some muscle fibers but otherwise very tendinous. On the right (Figure 4) some distal muscle fibers inserted onto the metacarpal shaft. In a second bilateral case (HU 708, 709) of sheet-like morphology the muscles are both proximally tendinous . On the right, about half way down the metacarpal shaft muscle fibers originating from the flat tendon merge with the much larger radial head of the first dorsal interosseous. In another bilateral case HU 712, 713) on the left, which is also doubled, the medial slip is a flat, tendinous sheet-like structure; the right is tendinous in the palmar view and with some muscle fibers in the dorsal view. The bilateral sheet-like case (Hu 730, Figure 3a and b, HU 731; doubled on the left and tripled on the right) also exhibit sheet-like morphology. In each hand, two separate, thin, flat, distal slips of muscle originate from a long, sheet-like tendon continuous from the proximal origin of the first dorsal interosseous radial head along about three-quarters the length of the metacarpal shaft. Of the unilateral cases, the right hand (HU 714) was a very flat, tendinous sheet which gave way mid-shaft to muscle fibers, many of which inserted right back onto the metacarpal shaft. The left hand (HU 718) presents a very diminutive example of interosseous palmaris I of Henle; a very thin sheet of muscle fibers lies right on the volar surface of the proximal metacarpal I shaft. About one - third of the way along the shaft it gives way to an even thinner and very delicate tendon disappearing deep to the muscle fibers of contrahens I which, in this case, also insert across the volar aspect of the distal metacarpal I shaft deep to the flexor pollicis longus tendon. Of additional interest in this case the deep head of Cruveilhier is doubled and the extensor pollicis longus tendon is doubled. In mentioned above. In another case (HU 719), an unpaired left hand, the interosseous palmaris I of Henle is one of the most reduced case we have observed: It was difficult to separate proximally from the contrahens I, sharing reduced muscular origin with the radial head of the first dorsal interosseous, a deep branch of the radial artery separates the origin from the contrahens I oblique head and distally a tendon inserts deep to the contrahens I.

Figure 5

The interosseous palmaris I of Henle was proximally tendinous in thirteen of the 30 hands. In a left hand (HU 729) of a pair where the muscle consists of three separate slips, all the slips are proximally tendinous and are clearly separate from the deep head of Cruveilhier and contrahens I. Additionally, a tiny tendon from the deep surface of the origin tendon of the deep head forms a small muscle slip which immediately divides into two slips; one inserts with an interosseous palmaris I of Henle slip on the distal shaft and the other inserts with the larger interosseous palmaris I of Henle. (See Figure 5).

Partial insertion of the muscle back onto metacarpal I is seen in six of 30 hands. Excepting the left hand (HU 729, Figure 5) of a pair, all these metacarpal I insertions occurred in muscles of sheet-like morphology. The exception, one of the hands where the muscle is tripled, the middle slip inserts by a tendon deep to the flexor pollicis longus tendon on the volar surface of the distal shaft. Additionally, this hand exhibits a doubled flexor pollicis longus tendon, albeit tiny, and a deep head of Cruveilhier in three separate muscle slips, the deepest of which inserts ulnarward with contrahens I and the adjacent interosseous palmaris I of Henle slip mentioned above. In Figure 4, the right hand (HU702) of a male, depicts the muscle inserting back onto the metacarpal shaft.

Figure 5

The interosseous palmaris I of Henle was difficult to separate from either the deep head of Cruveilhier or the contrahens I in twelve of the 30 hands. With the sole exception of HU707 noted above , these occurrences were all confined to the proximal one-third or less of the muscle. Distal separation was otherwise always seen.

Our Figure 6 (HU 728) depicts an unusual configuration of muscle slips which clearly illustrate the challenges of performing careful, accurate, minimally destructive dissections in the thenar compartment. All the drawings and photographs in the world are no substitute for a destroyed specimen. They also illustrate the difficulties interpreting others published dissections and illustrations. In this right hand, the interosseous palmaris I of Henle is of a normal configuration. Superficial to it is found a confusing array of muscle slips. Also (not shown) the flexor pollicis longus tendon is doubled. The tiny second tendon is not continuous, however. It commences from a tiny muscle belly off the main muscle about seven inches proximal to the flexor retinaculum forming a tendon five inches proximal to the proximal border of the retinaculum. The tiny tendon runs along with the large one into the thenar compartment area where it disappears as loose connective tissue only to reappear distally about one third of the way down the metacarpal I shaft level continuing on beyond the metacarpophalangeal joint along with the large flexor pollicis longus tendon.

Figure 6


We scored two characters lying very close to the interosseous palmaris I of Henle (Table 1 ). In eleven of 30 hands the deep head of Cruveilhier and the contrahens I inserted across the distal volar surface of the metacarpal I shaft and just deep to the large flexor pollicis longus tendon. Lastly, we encountered seven hands with doubled flexor pollicis longus tendons; only in the left hand of a female pair (HU 711) were the two tendons nearly the same size, all the other second tendons were very tiny .


DISCUSSION

All the debate surrounding the incidence, variation and even the existence of the interosseous palmaris I of Henle continues because this muscle is very deep in the thenar compartment. Moreover, it is usually very small and it appears to be quite variable in its morphology. Additionally, it is often closely apposed to the deep head of Cruveilhier, the radialward margin of contrahens I ( the oblique portion of the adductor pollicis ), and the first dorsal interosseous - - any or all of which it has been confused. Historically, the morphological confusion and ongoing discussion have been complicated and exacerbated by embryological and phylogenetic considerations often being brought to bear in attempts to resolve this complex issue. In the following paragraphs we seek to resolve this issue through: 1) discussion of our dissections on the 30 adult human hands reported on above; 2) consideration of our nonhuman primate hand muscle dissections (Dunlap et al., 1985; Dunlap and Aziz, 2012) ; 3) our dissections of human trisomies (Dunlap et al., 1986; Dunlap and Aziz, 2012) ; and 4) consideration of Cihak’s embryological insights (1972) along with other relevant published literature.

As discussed by Day and Napier (1961), the interosseous palmaris I of Henle first described by Jacob Henle in his 1841 Allgemeine Anatomie added a third muscle slip to the two headed “flexor pollicis brevis” of Albinus (1749). Day and Napier’s (1961) valuable discussion of the 19th and 20th century literature centers on the various identifications and descriptions of the flexor pollicis brevis. We will touch on some of this as we review several anatomy texts and papers focusing on the interosseous palmaris I of Henle. We regard Wood Jones (1942) book, “The Hand” , as an authoritative work. His chapter XXII, “The Intrinsic Muscles” , is the clearest exposition of the currently accepted anatomical description of the muscles including interosseous palmaris I of Henle. A “...first palmer interosseous...” is described as “... that slip of muscle that arises from the ulnar side of the basal portion of the metacarpal of the thumb and is inserted into the ulnar sesamoid of the metacarpo-phalangeal joint, and so into the ulnar side of the base of the first phalanx.” (p.267). Wood Jones makes no clear statement as to the incidence or variation of the muscle. In fact, no investigator we studied who clearly identifies the interosseous palmaris I of Henle ( Bryce, 1923; Jackson, 1925; Huber, 1930; Robinson, 1931; Grant, 1942; Wood Jones, 1942; Jamieson, 1946; Abramowitz,1955; Lockhart et al., 1959; Zuckerman, 1961; Breathnach, 1965; Mortensen and Pettersen, 1966; Lewis, 1965; Warwick and Williams, 1973; Gardner et al., 1975; Tobias and Arnold, 1977; O’Rahilly, 1983; Cartmill, 1987; Yamamoto et al., 1987; Ikebuchi et al., 1988; Zancolli and Cozzi, 1992; Perkins and Hart, 1993; Susman, 1999; Witthaut and Leclercq, 1999; Henkel-Kopleck and Schmidt, 2000; Standing, 2008; Morrison and Hill, 2011) has reported the range of variations we report here. Moreover, excepting Abramowitz, 1955; Lewis, 1965; Yamamoto et al., 1987; Ikebuchi et al., 1988; Perkins and Hart, 1993; Henkel- Kopleck and Schmidt, 2000, and Morrison and Hill, 2011 there is no clear statement for the incidence of the muscle.
Commencing with presence ( or absence ) as illustrated in our Table 1, we will discuss our findings. As stated above, we have found the interosseous palmaris I of Henle present in one form or another in all of our 30 hands. Relevant to incidence, Abramowitz (1955) reports the presence of the muscle in all 58 of his dissected human hands and Lewis (1965) also reports 100% presence in his 25 dissected hands. Morrison and Hill (2011) report the presence of the muscle in 41 of 45 dissected hands. Yamamoto et al. (1987) dissected 50 adult human hands from Okayama University Medical School cadavers. In describing the oblique head of adductor pollicis they identify 46 hands from which radialward slips originating from the shaft and base of metacarpal I insert on the wing tendon of the ulnar extensor expansion of digit I. Eight of these radialward slips also include fibers originating from the shaft and base of metacarpal II. In conclusion they regard these slips as “. . . the palmar interosseous muscle of the thumb.” (p, 103) . Ikebuchi et al (1988) found the muscle in all of the 25 human hands they dissected, including one with a distal metacarpal I attachment (p. 329). Perkins and Hart (1993) reported the presence of interosseous palmaris I of Henle in 68 of 80 hand dissections. In their study (which concentrated on identifying gross variations in tendons and muscles of the hand), they reported (without incidence) some origin variations, including: metacarpal I shaft origin, “...the capsule of the first carpometacarpal joint, or the trapezium.” (p. 227). Witthaut and Leclercq (1999) report finding interosseous palmaris I of Henle in sixteen of twenty dissections. Susman et al. (1999) report the muscles presence in 30 of 35 dissected human hands. Unfortunately Susman et al. (1999, p. 162) indicate that a portion of their sample was “. . . not dissected entirely by us, but were dissected previously during medical gross anatomy.” The incidence of the muscle in their sample should therefore be viewed with some caution. Cihak (1972) and Standing (2008) state that the muscle is variable. Warwick and Williams (1973) indicate there are no “.. gross variations...” (p. 554, 557 ). Lockhart (1959) states that the muscle is very small. Cunningham (1887) states the muscle is rarely missing; and some authors report the muscle may be missing (Bryce, 1923; Jackson, 1925; Grant, 1942; Mortensen and Pettersen, 1966 ; Standing, 2008).
Doubling is reported without indication of incidence by Bryce (1923) , Jackson (1925) and Grant (1942). In their sample of 81 hands Henkel-Kopleck and Schmidt (2000) found interosseous palmaris I of Henle in 56 hands; of those , the muscle was doubled in five cases and in three cases it was tripled.
Our third character state , a sheet-like structure, was not specifically identified by other investigators with the possible exception of Henkel-Kopleck and Schmidt (2000). However, we believe this sheet-like structure has been observed by many others and is simply included in the description variable or diminutive or even proximally tendinous (our fourth character state in Table 1). Henkel-Kopleck and Schmidt‘s (2000) elegant dissections identify a “ ligamentum metacarpale pollicis ”. They describe several manifestations of this ligamentous structure with or without the presence of the interosseous palmaris I of Henle. Without making any judgement about their functional significance for the “ ligamentum metacarpale” pollicis, we believe our sheet-like observations are equivalent anatomical structures. Their dissections are some of the very best we have seen in the literature from the past century. If we were to include Henkel-Kopleck and Schmidt’s “ligamentum” as a manifestation of the interosseous palmaris I of Henle , then the incidence of the muscle in their sample of 81 would probably be 100%. They do discuss the possibility of the muscle appearing in degenerate states. Also, their count of doubled and tripled muscles would undoubtedly increase with inclusions of their “ligamentum” as a manifestation or degeneration of the muscle because they report seeing the “ligamentum” in 32 hands which also had an interosseous palmaris I of Henle. Also based on our examination of the literature, Huber’s (1930) description comes the closest to describing this character state (sheet-like): “Occasionally it is so reduced in size as to appear to be wanting.” (p. 613). Our case HU 719 is clearly the most “wanting” example in our sample. It consists of a stout , short tendon originating from the ulnarward base of the metacarpal I shaft closely associated with originating fibers of the first dorsal interosseous which uncharacteristically merge with the radial margin of the contrahens I. This stout tendon tapers into a space filled with fat and loose fascia but also arching over to join the radial margin of the contrahens I. The origin of the remainder of the radial head of the first dorsal interosseous is very diminutive, not even extending half way down the shaft. Moreover, the ulnar head of the first dorsal interosseous off the metacarpal II shaft is very thin and flat contributing very little to the muscle insertion. Along the volar and ulnar margins of the metacarpal I shaft nothing was encountered other than fat, loose connective tissue and the princeps pollicis artery. Distally, along the ulnar side of the metacarpal I shaft another short, stout tendon is encountered just ulnarward from the artery and joining the tendon of contrahens I. This is the distal remnant of the interosseous palmaris I of Henle . Another description in the literature , not inconsistent with our observations of this most “wanting” example, is in Quain’s Anatomy (Bryce, 1923) where the interosseous palmaris I of Henle is described as occasionally arising “... from the ulnar side of the first metacarpal bone, but sometimes extends to the base of the second, or receives fibers from a fibrous arch bounding the gap for the radial artery in the first dorsal interosseous muscle.” (p. 154). Finally, Jamieson (1946) depicts a greatly reduced interosseous palmaris I of Henle in his plates 26 and 34.

Thirteen of our cases are proximally tendinous (see Table 1) and nine are included , not surprisingly, in the previous character state , sheet-like. These proximally tendinous examples include a distal muscle belly and/or significant muscle fibers also originating from the metacarpal I shaft, the deep head of Cruveilhier or the first dorsal interosseous radial head. In the literature Lewis (1965) states that often there is “... a slender proximal tendon...” (p. 283). Bryce (1923) hints at this condition in the quote directly above from his page 154.
Six hands exhibit partial insertion onto the metacarpal I shaft.
Twelve of our 30 hands exhibit an interosseous palmaris I of Henle which was difficult to separate from the contrahens I or the deep head of Cruveilhier. This condition was only encountered at or near the origin, with the exception of the left hand of a pair (HU 707) in which separation was difficult the entire length. Lewis (1965) found two of his 25 hands in which the interosseous palmaris I of Henle was “... not distinguishable from the M. adductor obliquus ...” (p. 183). Abramowitz (1955, p. 271) reports that in the majority of his hands the muscle was “... contiguous to the radial portion of the adductor pollicis obliquus.” but, “... a line of cleavage is always present, ...”
Also eleven of our 30 hands exhibit insertions of the contrahens I and/or the deep head of Cruveilhier across the distal volar surface of the metacarpal I and deep to the flexor pollicis longus tendon. Finally, seven of our cases exhibit doubled flexor pollicis longus tendons. The “second” tendon in case HU 706 is one of the smallest and could only be traced proximally as far as the fascia deep to the proximal border of the flexor retinaculum. Doubling of flexor pollicis longus is mentioned in “ Morris‘ Human Anatomy” (Jackson, 1925; Grant, 1942) .

Having the advantage of Cihak’s (1972) work on human ontogeny allows us to present a more complete explanation for the relationship of the interosseous palmaris I of Henle with the other deep thenar musculature, the first dorsal interosseous and the contrahens I. In his investigation of human interossei Cihak (1972) examined 86 hands from 10 to 75mm crown - rump length embryos. The interosseous palmaris I of Henle is the second flexor brevis profundus muscle of 10 across the deep palm. The muscle anlage from which arise these paired flexores breves profundi to each digit in most lower mammals appears at about 13 - to 14- mm crown - rump length in human embryos. The very first flexor brevis profundus (the most radialward extension of the deep palmar layer) is the opponens pollicis and the second gives rise to a portion of the opponens pollicis, both heads of the flexor pollicis brevis, the radial head of the first dorsal interosseous, and the interosseous palmaris I of Henle. Along with all the past discussion and speculation about the deeper layers of the palmar musculature, an interesting hint in the literature pointing the way to Cihak’s eventual embryological insight is found in an abstract by Straus (1946a) on intrinsic palmar muscle layers in tetrapod vertebrates. Straus includes the opponens digiti minimi (his opponens V) with the interossei laying deep to the contrahentes and cautiously, with a question mark, places the opponens pollicis with the most superficial layer along side abductor pollicis brevis, flexor pollicis brevis and the muscles of digiti minimi. Cihak’s embryological research (1972) shows that the opponens pollicis and the opponens digiti minimi are homologous respectively with the first flexor brevis profundus and the 10th flexor brevis profundus. Thus, it seems quite possible that Straus was thinking along the same lines. He further states that establishing muscle homologies across vertebrate classes is “... profitless and probably invalid.” (p.233). In 1946 this was sage advice since embryological investigations of the caliber of Gasser (1967) and Cihak’s team had not been undertaken. The interosseous palmaris I of Henle is the homologue of the primitive mammalian second flexor brevis profundus. If this is indeed the case, then we would expect to see a high incidence for the interosseous palmaris I of Henle and a quite variable muscle also . That is certainly the case in our sample of 30 hands. We believe this variability described above in our results section is a manifestation of the retention of a primitive structure from which significant selective pressure has been removed. In our sample (with some form of the muscle) its morphology ranged from the very tiny , thin sheet of fibers lying on the proximal palmar metacarpal I shaft with an even thinner, delicate tendon slip extending distally ( case HU 718) , through mostly tendinous and sheet-like muscles, doubled and tripled muscles and proximally tendinous ones. In the adult human, pollical flexion and adduction are carried out by much larger intrinsic muscles so it should not be surprising that the interosseous palmaris I of Henle is quite variable. Moreover, variability is seen in the muscle occasionally (six out of 30 hands) with partial metacarpal I insertion. More frequently , twelve out of 30 hands exhibited difficulty during dissection with separation from the contrahens I or the deep head of Cruveilhier . Possibly related to this last character state we also found eleven out of 30 hands in which either or both the deep head of Cruveilhier and the contrahens I inserted across the distal volar surface of the metacarpal I shaft deep to the large tendon of flexor pollicis longus. We believe this last condition was observed by Day and Napier (1961, page 126) where they stated: “In several instances the distal attachment of the deep head had spread from the radial sesamoid to reach the anterior aspect of the proximal phalanx.” Unfortunately they do not report an exact number of cases or a percentage. Their “several instances” are among the 53 of 65 hands with a deep head.

We have presented detailed findings on the interosseous palmaris I of Henle and believe they are not inconsistent with the published literature. Additional supporting evidence for accepting the strong likelihood of the interosseous palmaris I of Henle representing the second flexor brevis profundus found in many mammals is seen in ours and others dissections of human trisomies and nonhuman primates (See Table 2). In our publications ( Dunlap et al. , 1985, 1986; Dunlap and Aziz, 2012) details and illustrations may be found concerning the trisomy and nonhuman primate dissections. All our Hominoidea had interosseous palmaris I of Henle in some form or another, each looking very much like one of our 30 dissected hands and/or our trisomy hands. The Hylobates specimens each have normal muscles along with diminutive flexor pollicis longus muscles, the latter absent in our Pongidae. Turning to the relevant literature we find: Hartman and Straus (1933) in their description of the flexor pollicis in Macaca mulatta describe a “... deeper part of the deep head...” (p. 142) which easily could be

Chart

the interosseous palmaris I of Henle. Our M. nemestrina and all our Hominoidea possess interosseous palmaris I of Henle. Raven (1950, p.45) describes a deep head of flexor pollicis brevis inserting medially. This equivocal assignment is contrary to our observations since in very few instances have we found a deep head inserting medially. Sullivan and Osgood (1927) in their description of the flexor pollicis brevis (pp. 226 and 228) include a muscle slip which easily could be an interosseous palmaris I of Henle in “. . . the orang-utan, Simia satyrus.” ( p. 193). Abramowitz (1955) reports the muscle in his dissections of a Papio ursinus and a chimpanzee. St. John Brooks (1887), in describing the adductor pollicis obliquus and the flexor pollicis brevis in the chimpanzee, Pongo and Hylobates agilis includes slips which easily could be the muscle interosseous palmaris I of Henle. Lewis (1965) reports the presence of the muscle in Cebus nigrivittatus but not in Pan satyrus or Cercopithecus nictitans. This short, certainly not comprehensive , list indicates that the muscle is generally present in Hominoidea. Curiously, Susman et al. (1999, p. 162) “... did not observe a PPIM [interosseous palmaris I of Henle] in any of the monkeys or apes we dissected.” See Table 2. Mining the older literature, we find, can be quite fruitful. Most of these earlier investigators had wide experience in comparative anatomy and therefore their dissections and descriptions were accurate enough to reinterpret their often equivocal or inaccurate attributions given the more modern knowledge at our disposal from embryological studies, new dissections, primate behavior studies, genetics and an ever enlarging fossil collection.

Human trisomies manifesting developmental delay and retardation are natural experiments from which potentially valuable information may be obtained regarding evolutionary changes in muscle morphology and presence as well as specific developmental genetic information on effected chromosomes (Barash et al., 1970; Bersu and Ramirez-Castro, 1977; Aziz, 1979, 1980. 1981a, 1981b; Pettersen et al., 1979; Pettersen and Bersu, 1982; Opitz et al., 1979). From the muscle dissections performed to date on human trisomies it appears that a phenotypic specificity is expressed in the forelimb for trisomies 13, 18 and 21. From these observations trisomy 18 manifests the greater number of muscles which appear to enhance our appreciation of phylogenetic relations through the retention of nonhuman primate and mammal configurations, Interosseous palmaris I of Henle is one of those retained muscles, and its morphology and appearance in our sample compares well with the observations in our 30 hand dissections. Since developmental delay and retardation are so severe in all trisomies the range of variation is extreme: from absence in three hands to a left hand in a Ts 18 in which the “. . . flexor pollicis brevis superficialis was so diminutive that insertion was almost entirely on the ulnar side of the first proximal phalanx base. Since a portion of the origin was from the ulnar margin of the proximal metacarpal I, the muscle was indistinguishable from the interosseous palmaris I of Henle.” (Dunlap and Aziz, 2012) . These observations in trisomies are not inconsistent with normal human hand muscle development as examined by Cihak (1972), therefore substantiating his findings.

CONCLUSION

We believe our study demonstrates that the interosseous palmaris I of Henle is present in virtually 100% of the karyotypically normal Homo sapiens population. However, its’ manifestation is quite variable befitting reduced selective pressures. It is clearly derived from the flexor breves profundi layer, thus identifying it as the second of ten short flexors found in many mammals.
The presence of an interosseous palmaris I of Henle in Cebus (Abramowitz, 1955; Lewis, 1965; and Dunlap et al., 1985) to the exclusion of all other Platyrrhini (with the possible exception of Alouatta seniculus (Schon, 1968) is notable. The well known manipulative abilities of Cebus along with their pleasant social character has led to their use as helpful animals for quadriplegic and similarly handicapped humans. The other most manipulative and dextrous primates are of course the Hominoidea including Homo sapiens, all of which possess the interosseous palmaris I of Henle. Within the Hominoidea it is probably significant that the muscle is most abundantly present in Homo sapiens and possibly less so in the apes which have diminutive thumbs. The diminution of the thumb in the apes may be a derived condition , not characteristic of the common ancestor for Homo, and Pan. This appears to be supported by the fossil evidence for Ardipithecus ramidus from which most of the left hand bones were recovered and many of the right (Lovejoy et al., 2009).

ACKNOWLEDGMENTS

We are grateful to the families and individuals whose remains were donated to the Anatomy Department willed body program at Howard University College of Medicine. James Ferguson’s assistance in embalming and curation is much appreciated. We thank Marie T. Dauenheimer for the illustrations and science writer George Leopold for editing our manuscript. The first author accepts all responsibility for any errors in the text, illustrations or bibliography.

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