262 THE SALIVA FROM THE INDIVIDUAL GLANDS. 



The reflex center for the secretion of saliva is situated in the medulla 

 oblongata, at the origin of the seventh and ninth cranial nerves. The 

 center for the sympathetic fibers also is situated here. If the center 

 is directly irritated mechanically, as by pricking, salivation occurs; 

 suffocation has the same effect. This reflex may be inhibited by irrita- 

 tion of certain sensory nerves, as by drawing forward loops of intestines. 



The reflex center is in direct communication with the cerebral hemi- 

 spheres, as is evident from the fact that, with the thought of savory 

 substances, especially during the state of hunger, watery salivation takes 

 place. Irritation of the cerebral cortex, in the region of the cruciate 

 sulcus (Fig. 258) also causes a flow of saliva in the dog. Also central 

 disease in human beings may induce abnormalities in the secretion of 

 saliva through their influence upon the intracranial center. 



As long as all nerve-irritation is suppressed, no secretion of saliva 

 takes place, as, for instance, during sleep. Secretion likewise ceases 

 immediately after division of all of the glandular nerves. 



Inflammations of the buccal cavity, neuralgia involving the nerves of the 

 mouth, irruption of the teeth, ulcers of the mucous membrane, spongy conditions 

 of the gums (as from the long-continued use of mercury) often induce active 

 secretion of saliva (salivation, ptyalism), which rarely is unilateral. 



The parotid gland in the sheep (ruminant) secretes continually. Division of 

 all of the afferent nerves does not affect this secretion. Perhaps this gland con- 

 tains a center through which secretion is excited. 



Certain poisons also cause salivation by direct nerve-irritation, especially pilo- 

 carpin. Some, particularly atropin, paralyze the cerebral salivary nerves, and 

 thus cause a cessation of secretion. Administration of muscarin under these con- 

 ditions causes resumption of the secretion. Pilocarpin acts by irritation of the 

 chorda tympani. Administration of atropin during the resulting salivation 

 causes this to cease. Conversely, in the condition of abolished secretion of saliva 

 following the administration of atropin, pilocarpin or physostigmin causes a re- 

 sumption of the secretion. Curare acts as a sialogog by irritation of the center. 



THE SALIVA FROM THE INDIVIDUAL GLANDS. 



Method. For obtaining the isolated saliva from the individual glands a thin 

 metal tube is introduced into the excretory duct. If masticatory movements are 

 then performed, or if a pungent substance be placed upon the tongue, the saliva 

 will flow from the tube, drop by drop. 



Parotid saliva is not ropy, dropping readily, of alkaline reaction, with 

 a specific gravity of from 1003 to 1006. It contains 6.84 per cent, of 

 total solids, of which 3.40 per cent, are inorganic. On standing it be- 

 comes cloudy and precipitates, together with some globulin, calcium 

 carbonate, which is dissolved in fresh saliva as bicarbonate. 



Through the precipitation of calcium, salivary calculi may be formed in the 

 excretory ducts; dental calculi likewise may form, enclosing leptothrix- threads 

 and bacteria. 



Of the organic constituents of parotid saliva the most important is 

 ptyalin; mucin is absent. Saliva contains, further, small amounts of a 

 globulin-like body, alkali- albuminate and albumin, together with some 

 urea, traces of volatile acid, and it appears never to be free from potas- 

 sium or sodium sulphocyanid, which is wanting in some animals. 



This substance is recognized, after acidulating the saliva slightly with hydro- 

 chloric acid, by adding a solution of ferric chlorid, when, with the formation of 

 ferric sulphocyanid a dark red color results. Potassium sulphocyanid reduces 



