ii EXTERNAL DIGESTIVE SECRETIONS 81 



U. Louibroso, on injecting into the dog's jugular a large quantity 

 of pancreatic secretion collected directly from a Pawlow's fistula, 

 noted that the saliva did not (like other digestive secretions) 

 acquire any of the enzymatic properties characteristic of pancreatic 

 juice : bile, e.g., acquires an intense lipolytic activity, which 

 persists for several days after the injection. 



V. Chemical analysis shows the presence of a number of 

 proteins in the salivary glands, among them a nudeo- protein 

 (Hammarsten), a substance which forms a special enzyme known 

 as ptyalogen, or the zymogen of ptyalin (in the albuminous glands), 

 mucinogen, the mother-substance of nmcin (in the mucous glands), 

 and the mineral salts of blood serum. With prolonged stimulation 

 of the secretory nerves to the albuminous and mucous glands, both 

 the ptyalogen and the mucinogeu disappear to form again during 

 the resting period. 



The increase of volume and weight in the salivary glands 

 during rest depends largely upon the greater amount of proteins 

 absorbed, since this increases the nitrogen content. Glands sub- 

 jected to prolonged stimulation contain 7 per cent less solids than 

 the resting gland, but this difference depends partly on the 

 greater amount of water absorbed by the gland during secretion, 

 in analogy with the conditions that prevail during muscular work. 



As regards chemical composition we must distinguish the 

 mixed saliva or total secretion from the simple and compound 

 salivary glands that open into the buccal cavity, from the separate 

 salivas secreted from the albuminous, mucous, and mixed glands 

 respectively. 



Mixed saliva is colourless, with no smell, opalescent, viscid, 

 faintly alkaline in reaction ( = 0'097 per cent Na. 2 CO.j) or neutral, 

 its specific gravity being 1002-1006 in man (1007 in dog). It 

 is remarkable that the osmotic pressure of saliva is considerably 

 less than that of the blood. Nolf (1900) showed that the 

 saliva spontaneously secreted from the dog's submaxillary has 

 a freezing-point from -O'll to - 0'27, and that secreted during 

 stimulation of the chorda freezes at - 019 to - 0'4. The osmotic 

 pressure of dog's blood, on the contrary, corresponds to a lower- 

 ing of the freezing-point = - 0'549 to - 0'605 (see Vol. I. pp. 

 142, 148). 



If left to itself the mucin of the saliva precipitates, along with 

 the old epithelial cells thrown off by the buccal epithelium. It 

 also becomes turbid from the precipitation of the calcium carbonate 

 dissolved in the saliva in the form, of bicarbonate. The microscope 

 shows the so-called salivary corpuscles, which resemble small 

 leucocytes with granulations that exhibit lively Brownian 

 movements. 



It is not possible to obtain an exact estimation of the total 

 quantity of saliva secreted daily ; approximate figures only can be 



VOL. II G 



