296 A MANUAL OF PHYSIOLOGY 



been extended to other glands. The parotid of many, if not 

 all, mammals is a purely serous gland ; it secretes a watery 

 juice with a general resemblance in composition to dilute 

 blood-serum. The submaxillary of the dog and cat is a 

 typical mucous gland ; its secretion is viscid, and contains 

 mucin. The submaxillary gland of man is a mixed gland ; 

 mucous and serous alveoli, and even mucous and serous cells, 

 are intermingled in it (Plate II., Fig. i). The submaxillary 

 of the rabbit is purely serous. The sublingual is in general a 

 mixed gland, but with far more mucous than serous alveoli. 

 The mixed saliva is a somewhat viscous, colourless liquid 

 of low specific gravity (average about 1005), alkaline to 

 litmus, acid to phenolphthalein. Besides water and salts, 

 it contains mucin (entirely from the submaxillary, the sub- 

 lingual and the small mucous glands of the mouth), to 

 which its viscidity is due, traces of serum-albumin and 

 serum-globulin (chiefly from the parotid), and a ferment 

 ptyalin. The salts are calcium carbonate and phosphate 

 (often deposited as 'tartar' around the teeth, occasionally 

 as salivary calculi in the glands and ducts), sodium bicar- 

 bonate, sodium and potassium chloride, and almost always, 

 a trace of sulphocyanide of potassium, detected by the red 

 colour which it strikes with ferric chloride.* The total 

 solids amount only to five or six parts in the thousand. A 

 great deal of carbon dioxide can be pumped out from saliva, 

 as much as 60 to 70 c.c. from 100 c.c. of the 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. A very small quantity of oxygen (about 

 0*5 volume per cent.) appears also to be present even in 

 saliva which has not come into contact with the air 

 (Pfliiger). Under the microscope epithelial scales, 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 



* In ioo students the saliva only once failed to give the reaction, and 

 in this individual a trace of sulphocyanide was present 3 days later. It 

 is absent from the saliva of many animals. In 25 dogs submaxillary 

 saliva obtained by stimulation of the chorda tympani only once contained 

 a trace of it. 



