THE CARPUS AND THE TARSTTS. 31 



Ichthyosauria, where marginal bones are added, and some 

 of the digits bifurcate. 



The skeletal elements of the manus and pes are divi- 

 sible into a proximal set, constituting the carpus or tarsus ; 

 and a distal set, the digits, of which there are normally five, 

 articulated with the distal bones of the carpus and tarsus. 

 Each digit has a proximal hasi-digital(metacarpal, ov meta- 

 tarsal) bone, upon which follows a linear series of phalanges. 

 It is convenient always to count the digits in the same way, 

 commencing from the radial or tibial side. Thus, the thumb 

 is the first digit of the hand in man ; and the great toe the 

 first digit of the foot. Adopting this system, the digits 

 may be represented by the numbers i, ii, iii, iv, v. 



There is reason to believe that, when least modified, the 

 cai-pus and the tarsus are composed of skeletal elements 

 which are alike in number and in an-angement. One of 

 these, primitively situated in the centre of the carpus or 

 tarsus, is termed the centrale ; on the distal side of this are 

 five carpalia, or tarsalia, which articulate with the several 

 metacarpal or metatarsal bones ; while, on its proximal side, 

 are three bones — one radiale or tibiale, articiilating with the 

 radius or tibia ; one ulnar e or fibular e, with the ulna or fibula ; 

 and one intermedium, situated between the foregoing. Car- 

 pal and tarsal bones, or cartilages, thus disposed are to be 

 met with in some Amphibia and Chelonia (Fig. 11), but, com- 

 monly, the typical arrangement is distui'bed by the suppres- 

 sion of some of these elements, or their coalescence with 

 one another. Thus, in the carpus of man, the radiale, in- 

 termedium, and ulnare are represented by the scaphoides, 

 lunare, and cuneiforme respectively. The pisiforme is a 

 sesamoid bone developed in the tendon of the flexor carpi 

 ulnaris, which has nothing to do with the primitive carpus. 

 The centrale is not represented in a distinct shape, havino- 

 probably coalesced with one of the other elements of the 

 carpus. The fourth and fifth carpalia have coalesced, and 

 foi-m the single unciforme. In the tarsus of man the astra- 

 galus represents the coalesced tibiale and intermedium; the 

 calcaneum, the fibulare. The naviculare is the centrale. Like 



