332 ANIMAL PHYSIOLOGY. 
system are correlated to one another. The compound stomach 
of the ruminants, with its slow digestion of a bulky mass of 
food, which must be softened and thoroughly masticated be- 
fore the digestive juices can attack it successfully, harmonizes 
with the powerful jaws, strong muscles of mastication, and 
grinding teeth; and all these in marked contrast with the 
teeth of a carnivorous animal with its simple but highly effect- 
ive stomach. Compare figures in earlier pages. 
Mastication in man is of ‘that intermediate character befit- 
ting an omnivorous animal, The jaws have a lateral and 
forward-and-backward movement, as well as a vertical one, 
though the latter is predominant. The upper jaw is like a 
fixed millstone, against which the lower jaw works as a nether 
millstone. The elevation of the jaw is effected by the mas- 
seter, temporal, and internal pterygoid muscles; depressed by 
the mylohyoid and geniohyoid, though principally by the di- 
gastric. The jaw is advanced by the external pterygoids; 
unilateral contraction of these muscles also produces lateral 
movement of the inferior maxilla, which is retracted by the 
more horizontal fibers of the temporal. 
The cheeks and tongue likewise take part in preparing the 
food for the work of the stomach, nor must the lips be over- 
looked even in man. The importance of these parts is well 
illustrated by the imperfect mastication, etc., when there is 
paralysis of the muscles of which they are formed. Even when 
there is loss of sensation only, the work of the mouth is done 
in a clumsy way, showing the importance of common sensation, 
as well as the muscular sense. 
Nervous Supply.—The muscles of the tongue are governed by 
the hypoglossal nerve; the other muscles of mastication chiefly 
by the fifth. The afferent nerves are branches of the fifth and 
glosso-pharyngeal. It is, of course, important that the food 
should be rolled about and thoroughly mixed with saliva (in- 
salivation). 
Deglutition.—The transportation of the food from the mouth 
to the stomach involves a series of co-ordinated muscular acts 
of a complicated character, by which difficulties are overcome 
with marvelous success. 
It will be remembered that the respiratory and digestive 
tracts are both developed from a common simple tube—a fact 
which makes the close anatomical relation between these two 
physiologically distinct systems intelligible ; but it also involves 
difficulties and dangers. It is well known that a small quantity 
