Wyman.l 274 [June 5, 



While the homology of tarsal and carpal bones, as groups, is obvi- 

 ous, that of individual bones is quite difficult to determine. The fact 

 that in mammals the bones of these two groups are not conformably 

 placed in the two limbs, and in addition to this the constant variations 

 in the vertebrate series of the form and number of the pieces, some- 

 times reduced to two, as in the tarsus of a frog, and sometimes in- 

 creased to eleven, as in the cai'pus of the armadillo, renders the 

 probability of a satisfactory result being reached in the direction of 

 S23ecial homology well-nigh hopeless. The homologies, in man and 

 mammals, of the individual bones in the tarsus and carpus hitherto 

 determined, rest largely on their physiological correspondence. The 

 pisiform and the calcaneum, for example, are regarded as homotypes, 

 because each, in certain animals, as the Carnivora, has a large tuberos- 

 ity, and each is a lever for increasing the muscular power applied to the 

 motions of the foot. But in these animals they are highly specialized 

 parts, and are the farthest removed from the typical conditions which 

 are most generally the best represented in the lowest animals of a 

 given group. If anatomists had begun their studies with reptiles, such as 

 Plesiosaurus and Icthyosaurus, or with lizards and turtles, the homolo- 

 gies now generally recognized would not have been so persistently 

 brought forward. 



The mammalian foot, which connects with the leg solely by the 

 astragalus, gives one quite a different idea of the tarsus and its rela- 

 tions, from that derived from the same segment in reptiles, where, for 

 the most part, the astragalus articulates chiefly with the tibia, the os 

 calcis with the fibula, and a third bone is interposed between them. 

 Furthermore, the os calcis, which is so highly specialized in mammals 

 as to be distinguished at sight, has in reptiles the appearance of the 

 other tarsal pieces, and resembles a cuneiform bone. Even in mam- 

 mals the pisiform, as in man, is so reduced that it becomes relatively 

 insignificant, which circumstance, and its relation to the tendon of the 

 ulnar flexor, led Cruveilhier to class it among the sesamoid bones. 



In the ideal vertebrate skeleton the tarsal and carpal bones having 

 no special development, would be represented by two rows of polyg- 

 onal or circular discs, all alike, as is actually the case in the marine 

 saurians. Reducing all the bones in question to one and the same 

 form, as in the lowest groups, form would cease to be a guide to the 

 determination of the homology of any particular bone of the tarsal or 

 carpal series. Those parts will be homologous which occupy similar 

 and symmetrical positions ; the inner bone of the wrist articulating 

 with the ulna, will be homologous, or the homotype of the inner bone 

 of the tarsus articulating with the tibia ; or in other words, homolo- 

 gous bones are determined by the principle of symmetry and relative 

 position, and not by their teleological relations. 



