744 PHYSIOLOGY 



gland with demilunes, while the retro-lingual and sub-lingual gland when present 

 are of the mixed type. In the rabbit the sub-maxillary gland is serous, while 

 the sub-lingual gland is mucous. In the cat the sub-maxillary is mucous, the 

 retro-lingual is mixed, and the sub-lingual, when present, is mixed, with pre- 

 dominance of the mucous type. 



The normal behaviour of the salivary glands during digestion 

 is best studied in a method used long ago by De Graaf and reintro- 

 duced with considerable elaboration of late years by Pawlow. It 

 is possible without any disturbance of the animal's nutrition to 

 transplant the papilla on which the duct opens to the outside so 

 that the saliva from any particular gland shall flow externally instead 

 of into the cavity of the mouth. By attaching a small funnel to the 

 fistulous opening, it is possible to collect the pure saliva unmixed 

 with the secretion of any other of the glands. 



By this method it has been found that as soon as food is intro- 

 duced into the mouth there is a secretion of saliva, the relative extent 

 to which different glands are involved varying according to the 

 nature of the stimulation. Thus with meat there is only a small 

 amount of secretion, which is derived chiefly from the sub-maxillary 

 and sub-lingual glands, and is rich in organic constituents. When 

 dry material, such as dry powdered meat, is introduced the flow of 

 juice is more copious and more watery. The same effect may be 

 produced in the dog by psychic excitation. Thus salivation may 

 be induced by showing food to the dog, or even by the suggestion 

 that dry powder is to be introduced into the mouth. A comparison 

 of the juices obtained from different glands shows that the serous 

 and mucous glands differ, as might be expected, in the nature of 

 their secretion. A serous gland, such as the parotid, gives a thin 

 watery secretion almost free from mucin, but containing small traces 

 of coagulable protein. The mucous gland delivers 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 per- 

 centage 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 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. 



