DIGESTION 167 



cat, dog, horse, sheep, and ox the action is very feeble or entirely 

 absent. Meade Smith* states that the saliva of the horse will 

 convert crushed raw starch into sugar in fifteen minutes, and 

 that the process is continued in the stomach ; he further adds 

 that the saliva of the horse will convert cane into grape sugar. 

 In ruminants he believes starch conversion takes place both in 

 the mouth and rumen. Though we do not accept these views, 

 we shall shortly endeavour to show how starch is converted into 

 sugar in the stomach of the horse. It is interesting in this 

 respect to note that in man starch conversion, brought about by 

 the action of ptyalin, is also now recognised as taking place in 

 the stomach from the swallowed saliva — in fact, that the bulk of 

 the conversion must necessarily take place there, and not in the 

 mouth. 



Secretion of Saliva. — The mechanism concerned in the secre- 

 tion of saliva deserves careful attention, for the reason that it 

 throws considerable light on other secretory processes. The 

 subject has been worked out by so many competent observers 

 that the leading points are beyond all doubt ; the submaxillary 

 gland of the dog has mainly afforded the desired information, 

 and there is reason to believe that the same process holds good 

 for the parotid and other glands, both of this animal and of 

 herbivora. 



The chief point in the secretion of saliva is that it is controlled 

 by the nervous system, and is not directly dependent upon any 

 mere increase in the blood-pressure in the gland. Afferent 

 nerves — viz., the gustatory division of the fifth and the glosso- 

 pharyngeal — convey from the mouth to the medulla a certain 

 impulse, which, by means of efferent nerves, is conveyed to the 

 gland, and secretion results. The efferent nerve of the sub- 

 maxillary gland of the dog is supplied by the chorda tympani, a 

 small branch given off by the seventh cranial nerve, which enters 

 the gland at its hilum, and supplies the vessels with dilator and 

 the cells with secretory fibres. The second nerve supplying the 

 submaxillary gland is a branch of the sympathetic, which spreads 

 out and invests with constrictor fibres the walls of the artery 

 supplying the part (Fig. 59). Thus the chorda tympani supplies 

 the gland with secretory fibres and the walls of the vessels with 

 dilator fibres, while the sympathetic supplies the vessels with 

 constrictor fibres, and only a few secretory fibres. 



If the tongue or the lingual branch of the fifth or glosso- 

 pharyngeal nerves be stimulated, secretion of saliva results ; if 

 the sympathetic nerve be divided and the tongue then stimu- 

 lated, secretion follows ; but if the chorda tympani be previously 

 divided, no secretion follows on stimulation of the tongue, lingual, 

 * ' Physiology of the Domestic Animals.' 



