DIGESTION IN THE MOUTH 665 



a secretion which is viscid from the presence of mucin, and contains also a 

 small trace of coagulable protein. Both in parotid and mucous saliva the 

 percentage of salts is very low, so that the freezing-point of these fluids 

 is considerably higher than that of the blood-plasma. In the dog the saliva 

 is free from any ferments. In man ptyalin the starch-splitting ferment- 

 is found in the saliva from both kinds of gland, though it predominates in that 

 obtained from the parotid gland. The total amount of saliva which may be 

 obtained varies, of course, in different animals. Each gland may, however, 

 in the course of the day give an amount of juice far exceeding, e.g. ten or 

 twelve times, its own weight. In man it is reckoned that over one litre of 



J.C.S. 



V.J. 



FIG. 323. Diagram of nerve-supply to sub-maxillary gland. 

 Sm.G, sub-maxillary gland ; N.L, lingual nerve ; Ch.T, chorda tympani ; 

 Sm.Gl, sub-maxillary ganglion ; Sm.D, Wharton's duct ; V. J, jugular vein ; 

 C.A, carotid artery; G.C.S, superior cervical ganglion; N.S, sympathetic fibres 

 ramifying on facial artery. (After FOSTER.) 



saliva may be formed every twenty-four hours, and in the herbivora, such as 

 the horse, the total diurnal production must amount to many litres ; 500 

 grammes of hay alone may evoke the secretion of a litre of saliva. 



The intimate dependence of the secretion of saliva on the events occurring 

 in the mouth shows that the activity of the salivary glands must be excited 

 by reflex means. The afferent nerves in this reflex are those that supply the 

 mucous membrane of the mouth, i.e. the fifth nerve and the glossopharyngeal. 

 The efferent channels of the reflex were discovered by Ludwig. Each one of 

 the large salivary glands receives nerve fibres from two sources, viz. from the 

 cerebro-spinal and from the sympathetic system. It is probable that 

 the cerebrospinal supply is derived always from one part of the cerebral 

 axis, namely, from the filaments which make up the nervus intermedius. 

 From this point they diverge in their course to the glands. The fibres to the 

 sub-maxillary, the sub-lingual, and the retro-lingual glands pass into the 

 facial nerve, and then from this nerve along the chorda tympani to the lingual 

 division of the fifth nerve. The lingual nerve passes below the duct, and 

 just before it crosses the two ducts of the sub-maxillary and retro- or sub- 

 lingual glands it gives off a small branch backwards, namely, the chorda 

 tympani, which runs along the sub-maxillary duct to be distributed to the 

 glands, and in its course gives off fibres also to the retro-lingual (Fig. 323). 



