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 papillae, 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 



