354 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



ordinarily become organs of support for the body, 

 and that in them more than in 

 other Vertebrates these ap- 

 pendages cease to lie along- 

 side the body, or in a plane 

 more or less parallel to its 

 long axis, and come to be set 

 in a vertical plane. This 

 change having been effected, 

 we note here, as elsewhere in 

 the organs of the animal body, 

 a reduction of superfluous 

 parts, and a consolidation of 

 what remains. In a large 

 number of Mammals, the 

 thumb (pollex) or great toe 

 (hallux), which never have 

 more than two phalanges, ex- 

 cept in some, though not all, 

 Cetacea, is completely lost. 

 Among the Ungulata the 

 tendency to a further reduc- 

 tion is seen in the sheep and 

 ox on the one hand, where 

 two digits persist, and in the 

 horse, where the whole weight 

 is carried by the middle or 

 third digit of each limb. The 

 historical evidence as to the 

 gradual reduction of the 

 second and fourth digits in 

 the horse may be regarded as 

 complete. (Compare Fig. 151.) 

 While reduction affects 

 the digits, consolidation is 

 more often seen in the metacarpal and metatarsal, 

 and carpal and tarsal bones ; the muscles that are 



igf. 150. Fore - arm and 

 Manus of the Round- 

 headed Dolphin. 



Radius ; u, ulna ; c, carpal 

 hones ; mi, first ; ?>i5, fifth 

 metacarpal ; i to v, digits. 



