DIGESTION IN THE MOUTH 



709 



all the higher mammalia is a typical serous gland, though here and there a 

 mucous cell may be occasionally seen. The orbital gland of the dog represents 

 practically one of the mucous glands of the general mouth cavity on a large scale. 

 The sublingual and submaxillary glands in man represent a third type. Most of the 

 alveoli are mucous in character. At the ends of the alveoli are seen crescent-shaped 

 cells between the mucin-distended cells and the basement membrane. These are 

 known as the demilune cells, or the Crescent cells of Gianuzzi. In some cases these 

 mucous alveoli with demilunes may be. found alongside of typical serous alveoli. 



FIG. 334. 



A, serous gland ; B, pure mucous gland from mouth, 

 o, ducts; /, fat-cells. 



(K6LLIKEE.) 



Thus in man the submaxillary gland is usually a mixed gland, the serous alveoli 

 predominating. The sublingual gland is also mixed, but with a predominance of the 

 mucous alveolr. In the monkey the submaxillary gland is almost entirely serous. 

 In the dog the submaxillary gland is a pure mucous gland with demilunes, while the 

 retrolingual and sublingual gland when present are of the mixed type. In the rabbit 

 the submaxillary gland is serous, while the sublingual gland is mucous. In ths cat 

 the submaxillary is mucous, the retrolingual is mixed, and the sublingual, when 

 present, is mixed, with predominance of the mucous type. 



The normal behaviour of the salivary glands during digestion is best 

 studied by aid of a method used long ago by de Graaf and rsintroduced 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 easy 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 introduced 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 submaxillary and sublingual 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 fiom mucin, but 

 containing small traces of coagulable protein. The mucous gland delivers 



