DIGESTION 297 



attaches to certain peculiar bodies, either spherical or of 

 irregular shape, that are seen in the viscid submaxillary 

 saliva of the dog or cat. They appear to be masses of 

 secreted material. The quantity of saliva secreted in the 

 twenty-four hours varies a good deal. On an average it is 

 from i to 2 litres. (Practical Exercises, p. 375.) 



Besides its functions of dissolving sapid substances, and 

 so allowing them to excite sensations of taste, of moistening 

 the food for deglutition and the mouth for speech, and of 

 cleansing the teeth after a meal, saliva, in virtue of its 

 ferment, ptyalin, is amylolytic ; that is, it has the power of 

 digesting starch and converting it into maltose, a reducing 

 sugar. In man the secretion of any of the three great 

 salivary glands has this power, although that of the parotid 

 is most active. In the dog, on the other hand, parotid 

 saliva has little action on starch, and submaxillary none at 

 all; while in animals like the rat and the rabbit the parotid 

 secretion is highly active. In the horse, sheep, and ox, the 

 saliva secreted by all the glands seems equally inert. A 

 watery or glycerine extract of a gland whose natural secre- 

 tion is active also possesses amylolytic power. 

 rY Starch- grains consist of granulose enclosed in envelopes 

 ]^ s of cellulose. Only the granulose is acted upon by ptyalin, 

 and hence unboiled starch, in which the cellulose envelopes 

 are intact, is but slowly affected by saliva. When starch is 

 boiled, the envelopes are ruptured, and the granulose passes 

 into imperfect solution, yielding an opalescent liquid. If a 

 little saliva be added to some boiled starch solution which 

 is free from sugar, and the mixture be set to digest at a 

 suitable temperature (say 40 C.), the solution in a very short 

 time loses its opalescence and becomes clear. It still, 

 however, gives the blue reaction with iodine; and Trommer's 

 test (p. 23) shows that no sugar has as yet been formed. 

 The change is so far purely a physical one; the substance in 

 solution is soluble starch. Later on the iodine reaction 

 passes gradually through violet into red ; and finally iodine 

 causes no colour change at all, while maltose is found in 

 large amount, along with isomaltose, a sugar having the 

 same formula as maltose, but differing from it in the melting 

 point of the crystalline compound formed by it with phenyl 



