10 SALIVA. 



mere attempt, in genuine sincerity, and apart from all pretended 

 modesty, it will not be denied that far greater service will be done 

 to the theory of animal juices as well as to physiological chemistry, 

 by an experimental criticism, than by the most careful collection 

 of all that bears upon the subject in literature. In our attempt to 

 sift the rich mass of materials presented to our notice, we shall 

 endeavour to abstain from all mere polemical criticism, and adhere 

 to facts only, which must ever constitute the only solid support of 

 natural science. 



SALIVA. 



THE saliva discharged from the mouth is not merely a mixture 

 of the fluids secreted by the different salivary glands, but also con- 

 tains, as an essential constituent, the buccal mucus, or the secre- 

 tion of the mucous membrane of the oral cavity. The mixed or 

 ordinary saliva is therefore by no means identical with the secre- 

 tions of the different salivary glands, from which it differs both in 

 its chemical characters and in its physiological action. 



The mixed saliva of man and of most of the mammalia 

 exhibits the following properties: it occurs as a rather turbid, 

 opalescent, or faintly bluish white fluid, which is somewhat viscid 

 and capable of being drawn out in threads, and is devoid of odour 

 and taste. After standing for some time, it deposits a mucous 

 greyish white sediment, which, when examined under the micro- 

 scope, is found to consist chiefly of pavement epithelium, often 

 united so as to form shreds, and what are termed mucus- 

 corpuscles, which are usually a little larger than pus-corpuscles, 

 and generally exhibit a large, lenticular, excentric nucleus, even 

 without the application of any special re-agents. The specific 

 gravity of mixed saliva is liable to variations even in the normal 

 state ; for its density is partly dependent on the quantity of mucus 

 that may be mixed with it, and partly on the greater or less 

 attenuation of the glandular secretion; its usual variations in 

 man are between 1*004 and 1*006; it may, however, in the 

 normal state rise to 1*008 or 1*009, or, on the other hand, it may 

 sink to 1*002. Normal saliva presents a more or less distinctly 

 alkaline reaction : it has no poisonous effect either on plants or 

 animals. 



There is scarcely any animal fluid in which it is of such im- 



