32 PHYSIOLOGY OF FARM ANIMALS [CH. 



feriueiit called ptyalin (not always present), and some inorganic 

 matter. The salts found in saliva are chiefly carbonate of lime, 

 alkaline chlorides, and phosphates of lime and magnesia. Cal- 

 cium salts in the saliva are responsible for deposition of 'tartar' 

 on the teeth. In human saliva potassium sulphocyanide is also 

 present. The gases of saliva are carbon dioxide, which is very 

 abundant, and traces of oxygen and iiitrogen. The ferment 

 ptyalin although present in human saliva and in the saliva of the 

 pig is absent from that of the dog, and its occurrence in the horse 

 and other herbivora is problematical. The function of ptyalin is 

 to act on starch and convert it into maltose which is a sugar. 

 Such starch -converting ferments are called diastatic or amylo- 

 lytic. Their action is permanently destroyed by a high tempera- 

 ture and inhibited by a low one ; it is also partly inhibited by 

 weak acids or alkalis and destroyed by a strong acid. The 

 chemistry of starch conversion by ptyalin is dealt Avith in the 

 volume on nutrition. 



As already mentioned the horse and ox and other herbivora 

 masticate on one side only at a time, and it is interesting to note 

 that the parotid glands in these animals to a large extent secrete 

 unilaterally, that is to say, that the gland on the side where 

 mastication is taking place secretes two or three times as much 

 saliva as the gland on the opposite side ; on the other hand the 

 submaxillary and sublingual glands secrete equally on each side. 



In addition to any ferment action which the saliva may 

 possess, this secretion is of undoubted use in mastication and in 

 deglutition. In the ox and sheep it is of use also in rumination. 



The secretion of saliva is under the control of the nervous 

 system, the process bemg brought about by a reflex mechanism. 

 The afferent nerves are connected with the mucous membrane of 

 the mouth, and the efferent nerves pass to the different salivarj^ 

 glands. Each gland has a double nerve sujoply, one from a 

 branch given off the seventh cranial nerve and called the chorda 

 tympani (in the case of the submaxillary and sublingual) or from 

 the auriculo-temporal (in the case of the parotid), and the other 

 from the sympathetic or thoracic autonomic system. There are 

 two sets of nerves in the chorda tympani and in the auriculo- 

 temporal, vaso-dilator and secretory. Stimulation of the chorda 

 promotes dilatation of the blood vessels supplying the gland as 

 well as secretion of saliva, but that the two processes are separate 



