130 MANUAL OF PHYSIOLOGY. 



the mouth. But no bacteria or other fungi exist in the ducts of 

 the glands or saliva taken from the ducts with the necessary 

 aseptic precautions. 



The ordinary mixed saliva may be easily collected bychewiug 

 some insoluble material, such as a bit of rubber tubing, and col- 

 lecting the fluid which the motion causes to be poured into the 

 mouth. 



The collection of the secretion of the different glands requires 

 more delicate methods. It may be collected separately by placing 

 a canula in the duct of each gland. 



The parotid saliva obtained in this way is found to have no 

 structural elements nor mucus, and is a thin fluid dropping easily, 

 and not capable of being drawn into threads. It contains some 

 serum albumin and globulin, potassium sulpho-cyanate, and pty- 

 alin. The portion first secreted is commonly acid, and it never 

 becomes strongly alkaline. Its specific gravity is 1003-1004. On 

 standing it becomes turbid from the precipitation of carbonate of 

 lime, which existed as bicarbonate. 



The submaxillary secretion is more strongly alkaline than that 

 of the parotid ; it contains structural elements and ruuciu, but is 

 not so viscid as the general mouth-fluid. 



The sublingual is much more viscid than either of the others, 

 and is more strongly alkaline, and contains much mucus and 

 many salivary corpuscles. 



The Method of Secretion of Saliva. Under ordinary 

 circumstances very little saliva is secreted, only sufficient being 

 poured into the mouth to keep the surface moist. When, however, 

 food is introduced into the mouth, and the process of mastication 

 commences, the secretion goes on more or less rapidly, according 

 to the stimulating or non-stimulating character of the food. 



The activity of a salivary gland is at once brought about by 

 means of special nervous agencies when a stimulus is applied to 

 the mouth. We know that the nervous mechanism with which 

 \ve have here to do, is what is called a reflex act. The stimulus 

 travelling from the surface of the mouth to the nerve centres is 

 reflected thence to the glands. We speak, then, of afferent 



