MAMMALS. 



361 



distal portions. The carpal and tarsal bones are in two 

 or throe rows, those of the distal row being opposite or alter- 



\vith the others. In the tarsus the os calcis and astragalus 

 are the most prominent, the former being the fibulare, the latter 



tibiale and intermedium (p. 176). The digits are typically 

 rive in number, but Pcdctcs presents structures usually inter- 

 preted as a sixth toe. The tendency is constantly towards re- 

 duction in the number of digits, disappearance being preceded 

 bv a reduction in length, in which 

 case the metacarpals are shortened 

 and are occasionally reduced to splint 

 bones. In certain groups there fre- 

 quently occurs a fusion of the two 

 middle metacarpals. The phalanges 

 in the digits never exceed three, ex- 

 cept in the whales. Mammals are 

 spoken of as plantigrade, digitigrade, 

 or unguligrade, accordingly as they 

 walk upon the whole metacarpal or 

 metatarsal region, as in the bear and 

 man ; or upon the distal phalanges, 

 as in the cats and dogs ; or, again, 

 upon the nails (hoofs), as in the 

 horse and cow. 



The most striking feature of the 

 nervous system of existing mammals 

 is the great size of the brain, and es- 

 pecially of the cerebrum and cere- 

 bellum, the former overarching twixt and mid brains and reach- 

 ing the latter. In the lower mammals the cerebral surface is 

 smooth, but in the higher it is marked by gyri and convolu- 

 the effect of which is to increase the amount of sur- 

 face and consequently of gray matter. 1 This great increase 

 of the cerebrum is largely an increase of the pallium, only 

 a small portion of which remains epithelial in character. In 



* By some authors the mammals are sub-divided into two groups, Ineducabilia. witli 

 smooth cerebra (Fig. 349), and Educabilia, in which the surface of the cerebrum is convo- 

 luted (Fig. 54 ;. 



FIG. 348. Fore (right) and 

 hind (left) feet of tapir. a, 

 astragalus; c, cuneiforme in 

 fore foot, calcaneum in hind 

 foot ; c" , c'", cuneiforme ; cb f 

 cuboid ; F, femur ; /, lunare ; 

 ;, magnum ; , naviculare ; 

 p, pisiforme ; /v j , radius ; s f 

 scaphoid ; /, trapezoid ; 7', 

 tibia; u, unciforme; l r , ulna. 



