056 



OF SECRETION. 



Fig. 255. 



Fig. 256. 



Lobule of Parotid gland of a new-born infant 

 injected with mercury. Magnified 50 diam. 



Distribution of Capillaries around the 

 follicles of Parotid Gland. 



a sufficient idea of the structure of these glands in Man ; the follicles are 



very minute, having a diameter only about 

 Fig. 257. three times greater than that of the capillary 



blood-vessels. Their development commences 

 from a simple canal, sending oft" bud-like pro- 

 cesses, which opens from the mouth, and lies 

 amidst a cellular blastema. As development 

 proceeds, the canal becomes more and more 

 ramified, and communicates with the enlarged 

 parent-cells of the blastema, which remain as 

 the terminal follicles of the branches of the 

 gland-duct ( 823). 



863. The Salivary secretion is by no means 

 necessarily constant ; being almost or com- 

 pletely suspended by cessation of the move- 

 ment of the masticator muscles and tongue, if 

 unexcited by any nervous stimulus. Hence 

 it is, that the secretion is checked during sleep ; 

 so that, if the mouth be kept open, its surface 

 is almost dried up by the atmosphere. The 

 mode in which the secretion is excited through 



the influence of the nervous system has already been considered ( 625, 626). 

 The quantity of Saliva formed during the twenty-four hours has been estimated 

 at about 15 or 20 ounces ; but on this point it is evidently impossible to speak 

 with certainty. The fluid obtained from the mouth is of a more viscous cha- 

 racter than the true saliva secreted by the glands ; being mingled with mucus. 

 The salivary fluid varies as to its chemical reaction ; being sometimes slightly 

 acid, and sometimes slightly alkaline ; but it is seldom precisely neutral. 

 According to Huenefeld, it will at the same time strike a blue colour with 

 reddened litmus-paper ; and turn blue litmus-paper red ; but the saliva ex- 

 amined directly before, and during, the act of eating, is always alkaline. It 

 seems probable that the acid reaction is due to the mucus of the mouth ; 

 which, at times when only a small amount of saliva is excreted, is not neu- 

 tralized by its alkali. The specific gravity of Saliva varies from 1'006 to 

 T009. It contains a small number of corpuscles, which seem to be partly 

 epithelium-cells from the mucous surface of the mouth, and partly the secret- 

 ing cells of the salivary vesicles. The solid matter contained in Saliva 

 amounts to from 10 to 13 parts in 1000. The animal principles of which 



Rudimentary Pancreas from Cod : 

 a, pyloric extremity of stomach; b, in- 

 testine. 



