THE DIGESTIVE SYSTEM 55 
and furnished with muscular walls, the fibres of which are so 
arranged as by their regular alternate contraction and relaxation to 
drive onwards the contents of the tube from the first to the second 
of these apertures. The anterior or commencing portion of this 
tube and the parts around it are greatly and variously modified in 
relation to the functions assigned to them of selecting and seizing 
the food, and preparing it by various mechanical and chemical 
processes for the true digestion which it has afterwards to undergo 
before it can be assimilated into the system. For this end the tube 
is dilated into a chamber or cavity called the mouth, bordered 
externally by the lips, which are usually muscular and prehensile, 
and supported by a movable framework carrying the teeth; the 
structure and modifications of which have been already described. 
The roof of the mouth is formed by the palate, terminating behind 
by a muscular, contractile arch, having in Man and some few other 
species a median projection called the uvula, beneath which the 
mouth communicates with the pharynx. The anterior part of the 
palate is composed of mucous membrane tightly stretched over the 
flat or slightly concave bony lamina separating the mouth from 
the nasal passages, and is generally raised into a series of trans- 
verse ridges, which sometimes, as in Ruminants, attain a con- 
siderable development. In the floor of the mouth, between the 
rami of the mandible, and supported behind by the hyoidean 
apparatus, lies the tongue; an organ the free surface of which, 
especially in its posterior part, is devoted to the sense of taste, but 
which also, by its great mobility (being composed almost entirely 
of muscular fibres), performs important mechanical functions 
connected with masticating and procuring food. Its modifications 
of form in different mammals are very numerous. Between the 
long, extensile, vermiform tongue of the Anteaters, which is 
essential to the peculiar mode of feeding of those animals, and the 
short, sessile, and almost functionless tongue of the Porpoise, every 
intermediate condition is found. Whatever the form, the upper 
surface is always. covered with numerous fine papille, in which 
the terminal filaments of the gustatory nerve are distributed. 
Salivary Glands.—The fluid known as the saliva is secreted by 
an extensive and complex system of glands discharging into the 
cavity of the mouth (buccal cavity), the position and relation of 
some of which are exhibited in the woodcut on the next page 
(Fig. 19). 
This apparatus consists of small glands embedded in the mucous 
membrane or submucous tissue lining the cavity of the mouth, 
which are of two kinds (the follicular and the racemose), and of 
others in which the secreting structure is aggregated in distinct 
masses removed some distance from the cavity; other tissues besides 
the lining membrane being usually interposed, and pouring their 
