580 MR. J. C. GALTON ON THE MYOLOGY OF 



this species of Sloth, as well as in the Anteater, differentia ble into two layers — an upper, 

 the longest, and a lower, the shorter portion. 



Extensor carpi radialis. — Arises fleshy from the supinator ridge, immediately below 

 the lowest fibres of origin of the supinator longus, and is overlapped at the lowest half of 

 its origin by the commencement of the extensor communis digitorum. After running 

 for about two-thirds of its course, the muscle crosses the radius, and lies in a wide 



shallow groove between the external edge of the bone and the mesial ridge, just in front 



of the lowest point of insertion of the pronator teres. Here, too, the muscle becomes 

 tendinous, and is crossed by the tendon of the extensor ossis metacarpi pollicis. 



The muscle, instead of having only one tendon (as in the specimen described by Prof. 

 Humphry l ), has two distinct well-developed tendons, of nearly equal size, the outer- 

 most being slightly the broadest. They are inserted into the radial sides of the bases of 

 the metacarpals of the index and middle digits respectively. 



There is a slight indication of bifidity in the fleshy portion of the muscle, before it 

 becomes tendinous. 



What may be regarded as the main tendon is continued into that branch which goes 

 to the middle digit. This, taken in conjunction with the presence of the " brevior " 

 branch (of human-anatomy nomenclature) only, in Prof. Humphry's specimen, may 

 indicate that the " longior " element is of secondary development. 



According to Meckel 2 , in the Ai, Anteater, and Armadillo this muscle is single at its 

 origin, but eventually splits into two bellies, each of which has a proper tendon of attach- 

 ment to the two first metacarpals. In the Marmot, Porcupine, Beaver, Agouti, and 

 certain other Rodents the muscle is double for the whole of its length. 



Among Marsupials, according to the above anatomist, the tendon of this muscle, in 

 the Kangaroo, splits into two divisions, the smaller of which goes to the metacarpal of 

 the index. In D idelphy s , on the contrary, this muscle is entirely in two divisions, which 

 terminate respectively at the second and third metacarpals. 



Extensor ossis metacarpi pollicis.— Arises from the lower two-thirds of the sti 

 ridge of the ulna, and from the space included between this and the radial border of the 

 bone-also from the interosseous membrane, and from the neck of the radius, close to 

 the proximal half of the inserted portion of the supinator brevis. The muscle passes 

 over a shallow groove in the outer half of the distal end of the radius, where it becomes 

 tendinous, the tendon overlying those of the extensor carpi radialis, and being overlain 

 in turn by the terminal expansion of the supinator longus over the wrist. It terminates 

 finally on the dorsum of the trapezium, which bone, though not a supporter of the index, 

 is strongly attached to its radial side by a ligament. 



There is no indication in the tendon of this muscle of that bifidity which takes place 

 m its seria homologue the tibialis anticus. In Vasypus, too, as I have described, the 



tendon of tins muscle is single 



r> 



o 



The tendon of this muscle in the Great Anteater, which is also single, is princinall J 

 inserted into the trapezium; but "quelques fibres," says Pouchet', "tendineuses — 



1 Loc. cit. p. 306. » Op. cit. p. 538. 3 o» cit n 23 . ™ « i • * , 



* *■ ' yp- ctt. p. j«j; see, too, pi. u. figs. 1 and 3. 



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