THE DIGESTIVE SYSTEM 



53 



in which all five digits arc developed ; but when the hallux is 

 wanting the internal cuneiform may be rudimentary or altogether 

 absent. The three cuneiform bones sup- 

 port respectively the first, second, and third 

 metatarsals, and the cuboid supports the 

 fourth and fifth ; they thus exactly corre- 

 spond with the four bones of the distal row 

 of the carpus. 



In addition to these constant tarsal 

 bones, there may be supplemental or 

 sesamoid bones : one situated near the 

 middle of the tibial side of the tarsus, 

 largely developed in many Carnivora and 

 Rodentia ; another, less frequent, on the 

 fibular side ; and a third, often developed 

 in the tendons of the plantar surface of 

 the tarsus, is especially large in Armadillos. 

 There is also usually a pair of sesamoid 

 bones on the plantar aspect of each meta- 

 tarso-phalangeal articulation. In the young 

 of the carnivorous genus Cryptoprocta there 

 may be a second centrale, which usually 

 coalesces with the ectocuneiform. 



The metatarsal bones never exceed five 

 in number, and the phalanges follow the 

 same numerical rule as in the manus, never 

 exceeding three in each digit. Moreover, 

 the first digit, counting from the tibial side, 

 or hallux, resembles the pollex of the hand 

 in always having one segment less than 

 the other dibits. As the function of the 

 hind foot is more restricted than that of the hand the modifica- 

 tions of its structure are less striking. In the Cetacea and the 

 Sirenia it is entirely wanting, though in some existing members of 

 the first-named order rudiments of the bones of both the first and 

 second segments of the limb have been detected, and a femur is 

 present in the Miocene Sirenian Halitherium. 



Fio. 18.— Bones of the right 

 Human foot. T, Tarsus ; M, 

 metatarsus ; Ph, phalanges ; c, 

 calcaneum ; a, astragalus ; cb, 

 cuboid ; n, navicular ; c 1 , inter- 

 nal cuneiform ; c-, middle cunei- 

 form ; c3, external cuneiform. The 

 digits are indicated by Roman 

 numerals, counting from the 

 tibial to the fibular side. 



IV. THE DIGESTIVE SYSTEM. 



General Considerations. — The search after the purpose which 

 every modification of structure subserves in the economy is always 

 full of interest, and, if conducted with due caution and sufficient 

 knowledge of all the attendant circumstances, may lead to important 

 generalisations. It must always be borne in mind, however, that 



