DIGESTION. 141 



Notwithstanding the simple character of the masticatory act, it is 

 one of great practical importance. If hurriedly or imperfectly per- 

 formed, it leaves the food in a crude and intractable condition, liable to 

 cause subsequent disturbance in the digestive process. It is a necessary 

 preliminary for the more complicated physiological changes to take 

 place in the remainder of the alimentary canal. 



Saliva. 



The saliva is a compound fluid, derived from the secretion of four 

 different glandular organs namely, the parotid, subniaxillary, and sub- 

 lingual glands, and the muciparous glandules of the mouth. The 

 glands have a general resemblance to each other 

 in structure, being composed of distinct irreg- 

 ularly ovoidal masses, more or less flattened 

 into a polygonal form by mutual compression. 

 These masses or lobules are connected with 

 corresponding branches of the salivary duct, 

 which penetrate into their interior, and there 

 divide into smaller tubes, each one of which ter- 

 minates in a rounded sac called the glandular 

 follicle or alveolus. The appearance presented 

 upon an injection of such a lobule is as if the 

 follicles were arranged in clusters, like grapes. 



, ,, , ,, ,, ,, ,. LOBULE OF PAROTID GLAM> 



around the ends of the smaller salivary tubes. O f newly-born iufant. in- 

 .21.) A more complete examination has iected with mercury. <w ag - 

 shown, however, that the follicles are simply 



the rounded extremities of tubular or sac-like offshoots from the sali- 

 vary tube ; and that it is the windings and prolongations of the tube 

 which constitutes the secreting follicles of the gland. The follicles are 

 in general about 50 mmm. in diameter, and are lined with glandular \ 

 epithelium cells, which cover their internal surface and nearly fill their 

 cavity ; so that there is often only a small space, toward the central 

 part of the follicle, containing a transparent fluid produced by the 

 secreting action of the cells. The cells, which are arranged in a 

 single layer, are finely granular bodies, about 15 mmm. in diameter, 

 each with an oval nucleus, situated toward the external part of the 

 follicle. They are closely packed together in various polygonal form>. 

 The salivary tubes or ducts, outside the follicles, unite into larger 

 branches, until they reach the principal excretory duct. They are lined 

 with cells which differ in form from those of the follicles, being elon- 

 gated and cylindrical, each with a nucleus situated about its middle 

 portion. It is probable that the epithelium of the salivary duct- 

 well as that of the follicles, takes part in the process of secretion ; 

 since Pfliiger has found that in sections of the gland, examined immedi- 

 ately after being taken out of the body, drops of transparent fluid 

 may be seen exuding from the ends of the cylindrical epithelium 

 cells into the cavity of the duct. The follicles and lobules are sur- 



