CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 319 



at all events when the flow is at all copious, are alkaline ; that is 

 to say the natural secretion is alkaline, but this may be obscured 

 by acid changes taking place in the fluid which has been retained 

 in the duct, possibly by the formation of an excess of carbonic acid. 

 On standing, the clear fluid becomes turbid from a precipitate of 

 calcic carbonate, due to an escape of carbonic acid. It contains 

 globulin and some other forms of albumin, with little or no mucin. 

 Potassium sulphocyanate may also sometimes be detected, but 

 structural elements are absent. 



Submaxillary saliva, in man and in most animals, differs from 

 parotid saliva in being more alkaline and, from the presence of 

 mucin, more viscid ; it contains salivary corpuscles, that is bodies 

 closely resembling if not identical with leucocytes, and, often in 

 abundance, amorphous masses. The so-called chorda saliva in 

 the dog, that is to say saliva obtained by stimulating the chorda 

 tympani nerve, (of which we shall presently speak), is under 

 ordinary circumstances thinner and less viscid, contains less 

 mucin, and fewer structural elements, than the so-called sympa- 

 thetic saliva, which is remarkable for its viscidity, its structural 

 elements, and for its larger total of solids. 



Sublingual saliva is more viscid, and contains more salts (in 

 the dog about 1 p.c.), than the submaxillary saliva. 



The action of saliva varies in intensity in different animals. 

 Thus in man, the pig, the guinea-pig, and the rat, both parotid 

 and submaxillary and mixed saliva are amylolytic ; the sub- 

 maxillary saliva being in most cases more active than the parotid. 

 In the rabbit, while the submaxillary saliva has scarcely any 

 action, that of the parotid is energetic. The saliva of the cat is 

 much less active than the above; that of the dog is still less 

 active, indeed is almost inert. In the horse, sheep, and ox, the 

 amylolytic powers of either mixed saliva, or of any one of the con- 

 stituent juices, are extremely feeble. 



Where the saliva of any gland is active, an aqueous infusion of 

 the same gland is also active. The importance and bearing of this 

 statement will be seen later on. From the aqueous infusion of 

 the gland, as from saliva itself, the ferment may be approximately 

 isolated. In some cases at least some ferment may be extracted 

 from the gland even when the secretion is itself inactive. In fact 

 a ready method of preparing a highly amylolytic liquid tolerably 

 free from proteid and other impurities, is to mince finely a gland 

 known to have an active secretion, such for instance as that of a 

 rat, to dehydrate it by allowing it to stand under absolute alcohol 

 for some days, and then, having poured off most of the alcohol, 

 and removed the remainder by evaporation at a low tempera- 

 ture, to cover the pieces of gland with strong glycerine. Though 

 some of the ferment appears to be destroyed by the alcohol a 

 mere drop of such a glycerine extract rapidly converts starch into 

 sugar. 



