344 



DIGESTION 



juices yielded by the digestive glands, and the mechanism of their 

 secretion. Since it is along the digestive tract that glandular action 

 is seen on the greatest scale, this discussion will practically embrace 

 the nature of secretion in general. And here it may be well to say 

 that, although in describing digestion it is necessary to break it up 

 into sections, a true view is only got when we look upon it as a 

 single, though complex, process, one part of which fits into the other 

 from beginning to end. It is, indeed, the business of the physiologist, 

 wherever it is possible to insert a cannula into a duct and to drain 

 off an unmixed secretion, to investigate the properties of each juice 

 upan its own basis; but it must not be forgotten that in the body 

 digestion is the joint result of the chemical work of five or six 

 secretions, the greater number of which are actually mixed together 

 in the alimentary canal, and of the mechanical work of the gastro- 

 intestinal walls. 



Saliva. The saliva of the mouth is a mixture of the secretions 

 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 sublingual, 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 

 tmtcous, 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 re- 

 semblance 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. 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. 24) almost neutral. Besides water and salts, it contains 

 mucin (entirely ^rom 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, which hydrolyses starch, and therefore 

 belongs to the group of amlyases or diastases. It differs somewhat 

 from the amylase of pancreatic juice. But the small differences 



