306 



ANATOMY OF VERTEBRATES. 



offers its most simple condition in the Horse, fig. 190. It is essen- 

 tially, as in Protopterus, a jointed ray, but every part is adap- 

 tively modified for special ends and reciprocal adjustment and 

 interplay ; in the monodactyle Mammal it is, in fact, the result of 

 simplification from a more complex ancestral condition of limb, in 

 reference to the application of that limb to a more vigorous kind 

 of locomotion. Viewing the framework of such limb, here, in 

 merely its archetypal relations, we remark that the supporting 

 arch is incomplete, as in most Mammals ; the pleurapophysis, a, h, 

 is expanded into a ' scapula,' with its coalesced h^emapophysis as a 

 ' coracoid ' process, ib. k. The first segment of the appendage is 

 modified as ' humerus,' a ; the second segment as ' radius,' o, with 

 which has coalesced the process s, u, developed in most Mammals 

 ' ulna.' In the blastema between the second and third ray have 



as 



been formed a cluster of ossicles called 'carpal,' ic, z,2,3; the 

 third segment, 4, 5, is a metacarpal, and with it are connected two 



Oning. 



Atcles. Hyajua. Megathere. Sloth. 



styliform appendages, 6, 7 ; the abortive remnants of other me- 

 tacarpals. Next follow the three terminal shorter segments of 

 the limb-ray called 'phalanges,' 13, u, 15 ; the whole forming the 

 * digit' which answers to the middle finger, iii, in the pentadac- 

 tyle foot of beast and in the hand of man. Gradational stej)s to this 

 perfect condition of ' hand ' are selected from the Mammals with 

 claws, in fig. 191. In the Unau or Two-toed Sloth {^Bradypus 

 didactylus), the digits which are functionally developed answer to 

 the second, ii, and third, iii, in Man; the fourth, iv, and first, i, 

 are represented by styliform beginnings of their metacarpals. 

 Tlie carpal ossicles include one, s, t, answering to the separate 

 scaphoid, s, and trapezium, t, in Man, a ' lunare,' /, and ' cunei- 

 form,' c, a ' trapezoides,' r/, supporting the metacarpal of the second 



