1899.] DB, A. KEITH ON^ THE CHIMPANZEES. 305 



progression. In both, the muscles of the fifth toe show a marked 

 tendency to become vestigial — a condition which occurs in Man, 

 and which Mr. Herbert Spencer believes to be due to the wearing 

 of boots ; but the retrograde changes are most marked in the 

 Grorilla. In 4 out of 11 Chimpanzees this digit received a tendon 

 from the extensor hrevis cliyitorum, a tendon found in only 

 1 Grorilla out of 8. The flexor brevis of this digit Avas absent in 

 3 Chimpanzees and fibrous in 11 ; it was absent in 3 Gorillas, 

 fibrous in 6, and muscular in 3. The flexor accessorius is equally 

 variable in both ; it was found in the feet in 6 out of 10 Gorillas 

 and in 6 out of 11 Chimpanzees. The origin of the flexor brevis 

 digitorum shows much variation in both animals, but the tendency 

 for a complete transference of the origin of this muscle from the 

 tendon of the long flexor of the foot to the tuberosity of the heel 

 is most marked in the Chimpanzee, a character in which it more 

 resembles Man than its congener. 



The better adaptation of the lower extremity of the Chiuipanzee 

 for a climbing-organ is seen in the extensive insertion of the semi- 

 tendinosus, gracilis, sartorius, and biceps to the fascia of the leg, 

 in the occasional slip from the adductor magnus to the inner head 

 of the gastrocnemius, and in the separation of the scansorius. The 

 scansorins is a segmention from the anterior border of the deepest 

 gluteal sheet, for the more complete flexion of the hip-joint. It 

 existed as a separate muscle in 6 out of 11 Chimpanzees and in 

 only 2 out of 8 Gorillas. The lower extremity is nearly equal in 

 length (sometimes longer) to the upper extremity ; in the Gorilla 

 it is always shorter ; but the proportion of the anterior and pos- 

 terior limbs varies considerably. 



Some well-marked features, related to their methods of locomo- 

 tion, distinguish the upper extremity of the Chimpanzee from the 

 Gorilla. The arm of the Chimpanzee is that of the brachiators, 

 anthropoids like the Orang and Gibbon, which use the arms as one 

 of the main organs of locomotion. The arm of the Gorilla 

 resembles more in its proportions that of the lower Apes, Both 

 the Chimpanzee and Gorilla agree in showing many retrograde 

 changes in the thumb. In neither is it a grasping-organ. The 

 flexor longus pollicis is vestigial in both ; in Gorillas it was re- 

 presented by a tendinous thread springing from the deep flexor of 

 the index digit in 2 ; in the remaining 10 it was completely absent 

 or represented by a piece of tendon in the thumb only. In 25 

 Chimpanzees it was present as a thread in 15, and in the remaining 

 10 it was completely absent or merely the terminal part of the 

 tendon was present. The retrograde change has made furthest 

 progress in the Gorilla. The short muscles that flex the thumb 

 have the same arrangement in both, except that the opponens 

 pollicis is better marked in the Gorilla. 



There are differences in the extensor muscles of the thumb. 

 The tendon of the extensor ossis metacarpi is much more com- 

 pletely divided into a carpal and a metacarpal part in the Chim- 

 panzee ; and while this tendon sent a slip to the proximal phalanx 

 of the thumb, as it always does in Man, in 4 out of 9 Gorillas, 



