THE PALMARIS LONGUS MUSCLE AND TENDON. A STUDY O F 1600 EXTREMITIES1 ARTHUR F. REIMANN, EDWARD H. DASELER, BARRY J. ANSON AND LINDSAY E. BEATON Department of Amtomy, Northwestern University Medical School, Chimoo. Illinois THREB FIGURES The palmaris longus is one of the most variable muscles in the human body. It is considered to belong in the class of retrogressive muscles (Bryce, '23). With the palmar aponeurosis the muscle represents the most superficial part of the primitive common flexor muscle of the fingers. Among vertebrates the palmaris is restricted to the mammals, where, in the,succession of mammalian orders, no regularity or progression of its presence or absence exists. At one end of the mammalian series, the muscle is absent as frequently in the species of Edentates as it is present (Windle and Parsons, 1899). The palmaris longus is regularly found in ruminants, pachyderms, rodents and carcivores, but is wanting in cetaceans, insectivores and cheiroptera. Although best developed in the former group of tetrapodes, it is still subject to a number of variations, the most notable being fusion with the subjacent flexor digitorum sublimis. In general, the muscle is best developed where the forelimb functions for weight-bearing and progression in a role similar to that of the gastrocnemius of the lower extremity. Among primates, the palmaris longus muscle is found in the lower groups, but in the higher apes it is again variably absent. Appearing well-developed and regularly present in the orangutan, the muscle is occasionally absent in the gibbon and chimpanzee (Kohlebrugge, 1897 ; Keith, l899), and frequently absent in the gorilla. Humphrey (1872) and LeDouble (1897) both consider this retrogression to be evidence of the gradual development of prehension, i.e., diversion of muscular power for the independent motion of parts of the manus. As if in response to such need, the flexor digitorum sublimis and flexor digitorurn profoundus are large at the expense of the palmaris longus and 1 Contribution no. 414 from the Anatomical Laboratory of Northwestern University Medical School. 495 496 ARTHUR F. REIMANN AND OTHERS of the flexor carpi ulnaris. Considered collectively, any of the above mammalian groups, in which the palmaris longus muscle is well developed, possesses small o r regressive superficial flexors of the digits. I n those groups in which the Aexor digitorum sublimis is in the ascendant, the palmaris longus muscle is likely to be regressive and susceptible to a number of variations. All of these variations of shape, position or attachment, have their distinct counterpart in the human forearm, so that most of the variations recorded hereinafter for man are comparable with the muscle o r its variations recorded for other mammals. Typically, the palmaris longus of men is a slender, fusiform muscle situated between the flexor carpi radialis and flexor carpi ulnaris, and overlying the flexor digitorum sublimis. Its chief area of origin is the medial epicondyle of the humerus. It inserts by means of a slender flattened tendon which expands distally into the apex of the triangular palmar aponeurosis and into the transverse carpal ligament. Departures from this regular type are fairly common; they will be described and figured hereinafter. MATERIALS AND METHODS The current study covers a 10-year period during which 1600 extremities were examined;2 of these 398 were recorded with no reference to body side or cadaver; 478 arms were recorded as right or left, but not by cadaver; 724 were recorded for both body-half and cadaver. Major variations of the palmaris were noted together with the incidence of its aberrant forms in 530 extremities. The specimens were adult male and female American whites and negroes (in approximate ratios of 10 : 1and 7 : 1, respectively). OBSERVATIONS AND DISCUSSION Variations in the palmaris longus may be classified as follows: complete agenesis; variation in the location and form of the fleshy portion; aberrancy in attachment at either extremity ; duplication or triplication; accessory slips; replacing elements of similar form or position. 1. Absence The palmaris longus, like the pyramidalis, psoas minor, and plantaris muscles is rather frequently a b ~ e n t . The ~ incidence of agenesis iiz a The authors gratefully acknowledge the generosity of Prof. 0. F. Kampmeier, Prof. T. T. Job, and Dean J. J. Sheinen who made available material in their laboratories at the Medical School of the University of Illinois, Loyola Medical School, and t h e Chicago Medical School, respectively. 3 T h e incidence of agenesis of the pyramidalis has been reported as 17.7% (Beaton and Anson, '39), and 17.4% (Ashley Montagu, '39). THE PALMARIS LOKGUS MUSCLE A N D TENDON 497 the present study is 12.8% (205 in 1600 arms). Gruber (1868, 1872) reports the incidence of absence of the muscle as 12,7% (178 in 1400 studied) ; Schwalbe and Pfitzner (1899), 20.4% (106 in 520) ; LeDouble (1897), 17.5% (91 in 520). The percentage absence in these lots combined is 14.3 (580 in 4040). Adachi (1909) reports among the Japanese an absence of the palmaris longus in 3.4% of arms (30 in 884), while Nakano ('23) reports an incidence of agenesis in the Chinese of 2.2% (2 in 95). Thus it is evident that agenesis is much more common in the Occidental than in the Oriental races. Analysis by sides of the same cadaver shows that bilateral symmetry is preponderant over asymmetry. The present authors encountered 30 instances of bilateral absence in 362 bodies (8.3%) ; absence on the right side m l y in 17 bodies (4.776) and on the left side only in 13 bodies (3.6%). Gruber encountered 40 instances of bilateral absence in 400 bodies (10.0%); absence on the right side only in 4 bodies (1.0%) and in 16 bodies (4.0%) absence on the left side. LeDouble found 27 instances of bilateral absence in 260 bodies (10.4%); absence on the right only in 15 bodies (5.8%) and on the left in 22 bodies (8.4%). Total incidence of bilateral agenesis, then, is 97 instances in 1022 bodies (9.5%); 36 times (3.5%) on the right and 51 times (5.0%) on the left. I n regard to the incidence of palmaris agenesis in relation to body sides, the following results are presented. I n the present series, in 1202 extremities so recorded, the palmaris was absent 76 times on the right as compared with 70 times on the left. I n 1400 arms examined by Gruber, the palmaris was absent 78 times on the right and 100 times on the left side. Le Double, in 520 arms examined, noted absence on the right in 42 cases as compared with 49 cases on the left. When the above figures are totaled, it is found that of 3122 arms examined, agenesis occurred on the right side in 196 cases, on the left side in 219 cases. Absence, then, is slightly more frequent on the left than on the right side. Schaeffer ( '09) and later Thompson ( '21), recording the presence and absence of the palmaris in living subjects (by inspeation of flexor tendons at the wrist) found similar frequencies of agenesis: 23.0% on the left (184 in 800 arms) and 16.3% on the right (403 in 2401 arms). Inaccuracies which are inherent in the method employed account for the higher percentages. Records of sex and racial differences indicate that the incidence of agenesis is considerably lower in negroes than in whites, and lower in males (of each race) than in females (Thompson and others). 498 ARTHUR F. REIMANN A N D OTHERS Differences in the incidence of agenesis in the series hereinbefore recorded, the predominance of bilateral agenesis and the more significant differences between body sides and between sexes cannot -be explained on the basis of chance (Thompson). This author, in a study of 102 families, presents evidence to indicate that the absence of the muscle is inherited as a Mendelian dominant factor that is possibly sex-linked and influenced by such modifying factors as related structures, body side and sex. 2. Variation in. f o r m I n the 530 arms examined for anomalies other than agenesis, the incidence of variations of all types is 9.0% (46 in 530 arms). Variations in form are commonly seen. The belly of the muscle is normally proximal in position (fig. l a ) . But it may be centrally placed (fig. 2a) or entirely distal (figs. lb, lc). I n addition, a digastric type occurs, in which proximal and distal muscle bellies are connected by a central tendon (1in 530 arms). All phases of muscular development are encountered from a type completely muscular from origin to insertion, to a form in which only a fibrous vestige remained. Either the muscle belly or its tendon may be bifid, or both parts may be split. When a cleft is present in the muscle, the tendons therefrom are likely to insert separately into the palmar aponeurosis (fig. 2b). These several variations in form constitute 50% of all anomalies observed (exclusive of agenesis). 3. Variation in attachmeizt Variations in the origin and in the insertion of the palmaris longus occur with fair frequency. Normally the muscle arises from the common flexor tendon attached to the medial epicondyle of the humerus, and descends to a palmar insertion (fig. la). When auxiliary attachments are present they come from the antibrachial fascia and the related intermuscular septa. Accessory or isolated origin may be derived from the lacertus fibrosus (fig. 2a), flexor carpi radialis, flexor carpi ulnaris or flexor digitorum sublimis (Macalister, 1867). The insertion of the palmaris is highly variable. I n place of its usual insertion, the tendon may loose itself, at mid-forearm level, in the antibrachial fascia (fig. 2c); it may become partially or totally attached to the fascia of the thenar eminence (figs. 2b, 2c), o r may blend with the expansion of the flexor carpi ulnaris insertion over the wrist and thus come to lie on either side of the median nerve. I n addi- THE PALMARIS LONGUS MUSCLE AND TENDON 499 Figs. l a to lc, tgpea of palmaris longus muscle. a, regular; b, duplication of muscle, the ulnar element possessing distal muscular p a r t ; e, palmaris with distally placed belly of broad insertion. t 4 pi m T H E PALMARIS LONOUS MUSCLE AND TENDON 501 tion, the authors have observed a few speciniens in which the tendon inserts into one o r the other of the carpal bones forming the walls of the carpal canal. Instances of insertion into the tendons of the flexor digitorurn sublimis, flexor digitorum profundus and flexor carpi radialis have been reported by LeDouble, but have not been encountered in the present study. The tendon may be bifid distally (figs. 2b, 2c), the extra slip inserting into any one of the above-named structures as reported by Macalister. The latter writer also noted an unusual palmaris muscle which, after attaching to the ulna at mid-forearm level, continued as a tendon into the transverse carpal ligament. An instance of insertion into the interosseus membrane was reported by Koster (1856). Although variations in attachment are, as just described, of strikingly different types, their combined incidence is low, being 6.5% (3 in 46). Origin from the flexor digitorum sublimis and insertion into the hamate bone are the commonest variations within the general category. 4. Duplicaliouz, etc. Duplication of the palmaris longus is of relatively common occurrence, being 4 cases in the 46 instances of variations encountered exclusive of a g e n e ~ i s . The ~ occurrence of duplication, in the present authors’ series, is 0.8% (4 in 530 arms); this is much lower than Gruber’s 3.1% (31 in 1000 arms). When duplication does occur, the muscle on the radial side of the forearm is usually in its normal situation. The accessory, or ulnar, element (muscle and tendon) is likely to be smaller than the radial component. Frequently the ulnar member of the pair possesses a distal (fig. l b ) , or intermediate muscle belly (Wood, 1866). 5. Accessory slips and substitute structures I n addition to instances of duplication, where each portion of the palmaris longus has the usual proximal and distal attachments, the single or double muscle may be augmented by accessory slips, with an incidence of 32.6% (15 in 46). The most common type in this group (4 of 15) is a small accessory muscle which arises from the palmaris tendon and inserts into abductor digiti quinti muscle belly (Macalister, ’ Gruber reported one case in which three palmaris longus muscles and tendons were present. The radial portion was normal, while the intermediate and ulnar divisions were rudimentary. They all arose from the medial epicondyle of the humerus; the palmaris proper inserted into the aponeurosis and into the carpal ligament, while the distal extremeties of the two smaller tendons united to form a common tendon with the palmaris longus proper over the wrist. 502 ARTHUR F. REIMANN AND OTHERS L e D ~ u b l e ) .A ~ similar slip, with insertion into the flexor digiti quinti, occurred once in the present series (fig. lb). Portions of the regular tendon may split off to insert broadly into the volar antebrachial fascia (fig. 2c). Macalister and Morrison ('16) describe a muscle, accessory to the abductor digiti quinti, whose fleshy portion lying in the distal forearm, arose by two heads, one from antebrachial fascia, the other through a tendon from the palmaris. Accessory muscles may have widely divergent origins and yet eventually come to insert into the palmar aponeurosis or neighboring carpal ligament. I n one specimen the auxiliary slip arose from the flexor digitorurn sublimis by a tendon fused to the undersurface of that muscle (fig. 3c). The muscle belly appeared distally between the flexor carpi radialis and palmaris longus, where it obscured the median nerve, finally inserted broadly into the transverse carpal ligament. An accessory slip arising from the ulnar tuberosity and coronoid process has been reported (Macalister, Wood). I n the current set of specimens an unusual accessory slip was observed, which, beginning as a thin tendon from the coronoid process, the radial tuberosity and the oblique line of the radius, ended as a short muscle belly at the proximal edge of the transverse carpal ligament and on the sheath for the tendon of the flexor carpi radialis (fig. 3b). This muscle conforms in certain respects to one originally described and named the radio-carpus muscle by Fano (1851). It has been since found to insert more regularly into radial carpal bones and second and third metacarpal bones (Gruber, Wood, Macalister), thus becoming an accessory flexor carpi radialis. Accessory tendons may arise as such from the antebrachial fascia and insert into the transverse carpal ligament. A muscle arising from the lower two-thirds of the ulna under the flexor carpi ulnaris for insertion into the transverse carpal ligament has been noted by Macalist er. I n the absence of the normal palmaris muscle, the palmar aponeurosis and the transverse carpal ligament may receive the tendons of substitute muscles. The presence of a slip from the flexor digitorum sublimis has been described by many authors. A specimen in the present series possessed, bilaterally, an unusual stout muscle that arose from the lateral edge of the radius in its middle third external to the flexor digitorum sublimis under the pronator teres (fig. 3a). Below, its rounded tendon passed under the transverse carpal ligament to the The accessory adductor flexorum carpi radialem (Gantzen, MacWhinnie) , passing from flexor carpi radialis tendon to abductor digiti quinti, is usually grouped among the variations of the palmaris. It is more accurately a variant and accessory of the former muscle. THE PALMARIS LONGUS MUSCLE AND TENDON 503 Figs. 3a to 3c, types, continued. a, short broad muscle replaeing palmaris longus (tendon followed to deep aspect of aponeurosis); b, auxiliary muscle with long tendon of ulnar and radial origins (flexor digitorum longus, otc., removed) ; c, auxiliary muscle with robust, distally placed, fleshy part (flexor carpi radialis retracted). 504 ARTHUR F. REIMANN AND OTHERS radial side of the median nerve, broadened in the palm to insert into the deep aspect of the palmar aponeurosis. This muscle is the rare palmaris profundus of Frohse and Fraenkel ('08). I n some respects it is similar to the radio-carpus muscle mentioned hereinbefore. A tendinous slip from the flexor carpi ulnaris (Macalister) and a muscular slip from the flexor carpi radialis (Wood), passing to the palmar aponeurosis, have also been regarded as substitute structures. The present authors have encountered a muscular slip and tendon which passed from the pronator teres to the palmar aponeurosis. Frohse and Fraenkel described an unusual bicipital muscle, one head of which arose from the ulnar tuberosity by a long thin tendon, the other having the attachments and form of their palmaris profundus. I n general it may be said that morphologic variations in the present study occur much more frequently on the right side (28 of 46) than on the left side (18 of 46). CONCLUSIONS A N D SUMMARY In an anatomical study of 1600 arms the authors encountered 205 cases of agenesis (incidence, 12.9%). I n 362 consecutive cadavers, bilateral absence was encountered much more frequently than absence only on the right or on the left, the ratio of absences being 30: 17 : 13 (8.3; 4.7; 3.6% respectively). I n 1202 extremities, the palmaris was absent 76 times on the right as compared to 70 times on the left. The incidence of anomalies of all types, exclusive of agenesis, was 46 in 530 consecutive arms. Variations in position and form constitute one-half of these (23 in 46). A more varied group of accessory slips and substitute structures were encountered 15 times in the set of 46 anomalies, while duplication of the palmaris was encountered 4 times, aberrancies of attachment 3 times. Variation of the several types, combined, was more frequent on the right side (28 times) than on the left (18 times), in marked contrast to the relation of agenesis to body-side. LITERATURE CITED ADACHI, B. 1909 Beitrage zur Anatomie der Japaner. Die Statistik der Muskelvarietaten Zweite Mitteilung. Zeitsch. f. Morphol. u. Anthropol., Bd. 12, S. 261-312. ASHLEY-MONTAGU, M. F. 1939 Anthropological significance of t h e musculus pyramidalis and its variability in man. Am. J. Phys. Anthrop., vol. 25, pp. 435-490. BEATON, L. E., AND €3. J. ANSON 1939 The pyramidalis muscle: its occurrence and size in American whites and negroes. Am. J. Phys. Anthrop., vol. 25, pp. 261-269. BRYCE, T. H. 1923 I n Quain's Elements of Anatomy. 11th ed. Longmans, Green, London. FANOQuoted by Macalister. FROHSE, F., AND M. FRAENKEL 1908 Die Muskeln des Menschlichen Armes. I n v. Bardeleben 's Handbuch der Anntomie des Menschen. Fiseher, Jena. T H E PALMARIS LONGUS MUSCLE AND TENDON 505 GANTZEN Quoted by LeDouble. GRUBER,W. 1872 Beobaehten aus der Menschlichen und Vergleichenden Anatomie. Berhn. 1868 Memories de 1’Academie Imperiale de St. Petersbourg. T. 11, pp. 1-26. HUMPHRP,G. M. 1872 The muscles of vertebrates. J. Anat. and Physiol., vol. 6, pp. 293-376. K E I ~ A. , 1899 On the Chimpanzees and their relationship to the Gorilla. Proc. Zool. SOC. London, pp. 296-314. EOHLEBRUQGE, J. H. F. 1897 Muskeln und peripheren Nerven der Primaten. Verhandl. der Konink. Akad. Wet., Amsterdam. Sect. 2, Bd. 5, S. 246. ROSTER Quoted by Macalister. LEDOUBLE,A. F. 1897 Traite des Variations du System Museulaire de L ’Homme. Schleieher, Paris. MACALISTER,A. 1866-69 Further notes on muscular anomalies in human anatomy and their bearing upon homotypical myology. Proc. Roy. Irish Acad., vol. 10, pp. 121-164. 1872-74 Additional observations on muscular anomalies in human anatomy. Trans. Roy. Irish Aead., vol. 25, pp. 1-134. MACWHINNIE Quoted by Wood. J. T. 1916 A palmaris longus muscle with a reversed belly, forming an accessory MORRISOH, flexor muscle for the little finger. Jour. Anat. and Physiol., vol. 50, pp. 324-326. NAKANO,T. 1923 Beitrage zur Anatomie der Chinesen. Die Statistik der Muskelvarietaten. Folia Anat. Jap., vol. 1, pp. 273-282. SCHAEFFER, J. P. 1909 On the variations of the palmaris muscle (abstract). Anat. Rec., vol. 3, pp. 275-278. SCHWALBE, G., A ND W. PFITZNER 1899 Varietaten-Statistik und Anthropologie. Morph. Arbeit., Bd. 3, S. 459-490. THOMPSON, J. W., 5. MCBAITS AND C. H. DANFORTH 1921 Heredity and racial variation in the musculus palmaris longus. Am. J. Phys. Anthrop., vol. 4, pp. 204-218. WINDLE,B. C. A., AND F. G. PARSONS 1899 On the myology of Edentata. Proc. Zool. Soe. London, pp. 210-221. WOOD,5. 1866-67 Variations in human myology observed during the Winter Session of 1866-67 a t King’s College, London. Proc. Roy. SOC.London, vol. 15, pp. 518-546.