a correct System of Muscular Homologies. 319 



by the assumption that for the typical digit there are four such 

 muscles — a palmar on each side and a dorsal on each side. In 

 the human manus, which we will take for illustration as the 

 most familiar, we have these developed as follows. The pollex 

 on its free side has a muscle, the abductor, which, however, is 

 generally divisible into two, an abductor exterior and interior, 

 so named by Sommerring. The interior of these, undoubtedly, 

 acts (as Meckel suggests) as a palmar interosseus ; this muscle 

 is present in Ornithorhynchus^ in the opossum, in the bear, 

 Guloj and others. On its ulnar side a muscle is occasionally 

 present as an anomaly in man, described by Henle — the 

 interosseus primus volaris, which I have never found as a 

 portion of the normal anatomy of any animal. Meckel, how- 

 ever, in speaking of the short muscles of the thumb, says : — 

 " II y a quelquefois, par exemple chez le magot, un petit fl^- 

 chisseur plus profond que Ton rencontre parfois aussi dans 

 riiomme." This might perhaps be the muscle of Henle ; 

 and Prof. Huxley has described it as existing in the gorilla 

 (Med. Times & Gazette, 1864, p. 538). For the index finger 

 there is a radial palmar muscle, which in the human subject 

 arises from the os magnum and the base of the third meta- 

 carpal ; but as its function as a radial lateralizer of the index 

 is better fulfilled by one of the dorsal muscles, its insertion is 

 shifted to the inner sesamoid bone of the pollex, and it becomes 

 the deep head of the flexor pollicis. I have found this muscle 

 in Hystrix cristata\ and, as stated above, in Dasy-pus this 

 muscle is present and typical, attached to the radial side of the 

 index. The palmar ulnar interosseus is developed as the first 

 palmar interosseus of the human hand. It Ls present in the 

 pes of many monkeys, of the dog, and the Ormthorhynchus. 

 For the middle finger the two palmar interossei, being super- 

 seded in function by the dorsals, would be entirely atrophied, 

 but that they are devoted to a special purpose; and hence, 

 coalescing, they are inserted into the inner sesamoid bone of 

 the pollex, constituting the adductor pollicis. In the dog this 

 muscle is represented by a fibrous band, not truly muscular ; 

 it is muscular, however, in Ursus arctos ; and in some monkeys 

 (as Macacus nemestrinus) it is large ; in Ursus a slip of it is 

 occasionally inserted into the second toe at its base, constituting 

 a special adductor indicis. Nycticehus presents us with the 

 intermediate state of this muscle, between the foot arrange- 

 ment, to be referred to presently, and the typical human ar- 

 rangement ; for in this animal the accomplished anatomists 

 who have given us so complete a monograph upon its myology 

 have described, besides the typical interossei, fine fasciculi 

 arising from the third and fourth metacarpals and inserted into 



