320 4 MANUAL OF PHYSIOLOGY 



secretion i.e., more than can be obtained from venous blood. 

 Only a small proportion of this is in solution, the rest existing as 

 carbonates. Oxygen is also present even in saliva which has 

 not come into contact with the air, and, indeed, in somewhat 

 greater quantity than in serum (about 0*6 volume per cent, 

 in dog's saliva). Under the microscope epithelial scales, dead 

 and swollen leucocytes (the so-called salivary corpuscles), 

 bacteria, and portions of food, may be found. All these things 

 are as accidental as the last they are mere flotsam and jetsam, 

 washed by the saliva from the inside of the mouth. But greater 

 significance 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. 422). 



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, 

 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. 



When starch is boiled, the granules are ruptured, and the 

 starch 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. 10) 

 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 some 

 isomaltose, a sugar having the same formula as maltose, but 

 differing from it in the melting-point of the crystalline com- 

 pound formed by it with phenyl-hydrazine (p. 488). Traces of 

 dextrose, a sugar which rotates the plane of polarization less 

 than maltose, but has greater reducing power, may be found 

 among the end-products when the digestion is conducted in 



