SECRETION OF SALIVA. 139 



more delicate methods. It may be collected separately by plac- 

 ing 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 sub-maxillary secretion is more strongly alkaline than that 

 of the parotid ; it contains structural elements and mucin, 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, how- 

 ever, food is introduced into the mouth, and the process of mas- 

 tication 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 

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

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

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

 nerves which carry the impulses toward the centre, and efferent 

 nerves which carry them from the centre. 



If we review the ordinary circumstances giving rise to a flow 

 of saliva, there will be no difficulty in determining the nerves 

 which act as the afferent channels in the simple reflex act. 



Stimulation of the mucous membrane of the tongue and mouth, 

 whether chemically, as with irritating condiments, or mechani- 

 cally, as by the motions of mastication, is generally transmitted 



