DIGESTION 319 



The Chemistry of the Digestive Juices. 



Saliva. The saliva of the mouth is a mixture of the secre- 

 tions of three large glands on each side, and of many small ones. 

 The large glands are the parotid, which opens by Stenson's 

 duct opposite the second upper molar tooth ; the submaxillary, 

 which opens by Wharton's duct under the tongue ; and the sub- 

 lingual, opening by a number of ducts near and into Wharton's. 

 The small glands are scattered over the sides, floor, and roof of 

 the mouth, and over the tongue. 



Two types of salivary glands, the serous or albuminous and the 

 mucous, are distinguished by structural characters and by the 

 nature of their secretion ; and the distinction has 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 sub- 

 maxillary 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. 

 The submaxillary of the rabbit is purely serous. The sublingual 

 is, in general, a mixed gland, but with far more mucous than 

 serous alveoli. Some of the small glands are serous, others 

 mucous in type. 



The mixed saliva of man is a somewhat viscous, colourless 

 liquid of low specific gravity (1002 to 1008, average about 1005), 

 alkaline to litmus, acid to phenolphthalein, but when tested by 

 the electrical method (p. 23) almost neutral. Besides water and 

 salts, it contains mucin (entirely from the submaxillary, the 

 sublingual 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 called ptyalin, which 

 hydrolyses starch, and therefore belongs to the group of amylases 

 or diastases. An oxydase or oxidizing ferment is also present. 

 The salts are calcium carbonate and phosphate (often deposited 

 as ' tartar ' around the teeth, occasionally as salivary calculi 

 in the glands and ducts), sodium bicarbonate, sodium and potas- 

 sium 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 



* In zoo students investigated by the writer 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 tym- 

 pani only once gave the ferric chloride reaction, and then faintly. 



