CHAP. XXIII.] THE SALIVA. 183 



cal an hypothesis seems quite superfluous,, since the nervous sympa- 

 thies which so evidently stimulate or control other secretions, as 

 the tears, are quite sufficient to explain this. The parotids pour 

 their secretions into that compartment of the mouth which is 

 outside the teeth, while the ducts of the submaxillary and sub- 

 lingual glands open under the tip of the tongue within the alveolar 

 arches. 



The salivary glands consist of a single excretory duct, continuous 

 with the mucous membrane^ branching again and again towards the 

 gland, so as to subdivide it into a multitude of lobes and lobules in- 

 vested with subdivisions of a common areolar or fibrous capsule, and 

 reducible ultimately to follicles of a highly delicate basement mem- 

 brane, lined by glandular epithelium, and provided oh their exterior 

 with a network of anastomosing capillaries. Some anatomists con- 

 sider that the ultimate follicles of these glands, in which the secretion 

 is elaborated, are at first closed sacs, in which the epithelium grows 

 and is multiplied, and which discharge themselves at stated inter- 

 vals into the extremities of the duct. Knowing how difficult it is 

 to determine the positive truth on this question, we shall merely say 

 that we are disposed to regard the secreting follicles as permanently 

 open to the duct, and their secretory epithelium as a continuation 

 of that which lines the duct. 



Salivary glands exist in all the vertebrata except fishes. 



The mere sight, or even the idea, of food to a hungry man, ex- 

 cites the salivary secretion " makes the mouth water" ; and during 

 mastication, it is poured very abundantly into the mouth, especially 

 at the commencement of a meal, or if the food taken is of a savoury 

 quality. Ordinarily, between meals and at night, these glands 

 hardly pour out any secretion ; but there can be little doubt that in 

 these intervals of comparative repose of their blood-vessels, the epi- 

 thelium of the follicles is undergoing those processes of growth and 

 change which precede the actual formation of the salivary fluid ; 

 and the same may be said of many other glands which have an 

 apparently intermittent action. The quantity of saliva furnished in 

 a given time in a state of health has been variously computed. 

 Mitscherlich collected between two and three ounces from the pa- 

 rotid duct in the course of twenty -four hours, and nearly fourteen 

 from the whole of the salivary organs ; and his estimate appears 

 worthy of being relied on. 



The saliva is a slightly viscid transparent fluid, depositing a little 

 flocculent sediment on standing, which consists principally of the 

 scaly epithelium of the mouth, and of other smaller nucleated cells, 



