72 ELEMENTS OF MAMMALIAN ANATOMY 



The poUcx or thumb has only two phalanges. The row of 

 phalanges articulating with the metacarpals is called the proxi- 

 mal, the terminal row the distal, and the remaining row the 

 middle phalanges. Two small sesamoid bones are attached on 

 the volar side of the junction of each proximal phalanx with 

 the metacarpal. Each distal phalanx has its proximal end 

 produced caudal on the palmar aspect so that its articular 

 surface faces dorsal. The distal phalanges terminate in claws 

 whih are retractile. 



The number of digits in the forefoot of Mammalia varies 

 from one to five. Nearly all the species of every order except 

 the Edentata and Ungulata possess five digits (pentadactyl). 

 The tapir has four functional digits (tetradactyl). The cow, 

 deer, sheep, and pig also have four digits, but only two are 

 functional (didactyl), the second and the fifth being atrophied 

 and terminating in the small caudo-lateral hoofs which do 

 not touch the ground. The rhinoceros has three functional 

 digits (tridactyl), and the Equida^ possess only one digit (mono- 

 dactyl) (Fig. 38), the third, though the atrophied remnants 

 of the second and fourth metacarpals are present as splint bones 

 which do not support phalanges. 



As before stated, the Ungulates are undoubtedly descendants 

 of a five-toed ancestor of Eocene times. The evolution of the 

 horse's limb and the reduction in the number of digits are shown 

 in figure 36. Fossil remains of the ancestors of the horse have 

 been found in western United States, Europe, and South 

 America. Orohippus lived in the region of Wyoming, Mon- 

 tana, and Idaho probably more than 5,000,000 years ago, 

 when that country was more or less marshy, and it was neces- 

 sary that the mammals should possess a spreading foot which 

 would not permit them to sink too deep into the mud. As the 

 ground become firmer and preying Carnivora more numerous, 

 the foot of the horse became adapted to rapid flight over solid 

 ground. Thus, through the process of evolution the third 

 digit w^as enlarged at the expense of the other digits. Confir- 



