THE DIGESTIVE SYSTEM 53 
in which all five digits are 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 C'ryptoprocta there 
may be a second centrale, which usually 
coalesces with the ectocuneiform. 
; The metatarsal bones never exceed five a rae ua ste ght 
in number, and the phalanges follow the smetatarsus; Ph, phalanges; c, 
same numerical rule as in the manus, never caleaneum; a, astragalus; cb, 
exceeding three in each digit. Moreover, ei Ee a 
the first digit, counting from the tibial side, form ; c3, external cuneiform. The 
or hallux, resembles the pollex of the hand ‘its are indicated by Roman 
. « numerals, counting from the 
in always having one segment less than tiyiai to the tibular side. 
the other digits. 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 Hulitherium. 
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 sutticient 
knowledge of all the attendant circumstances, may lead to important 
generalisations. It must always be borne in mind, however, that 
