DIGESTION 295 



Besides these unformed ferments, certain formed ferments, 

 or micro-organisms, are present in parts of the alimentary 

 canal, and even in normal digestion contribute to the 

 changes brought about in the food ; while under abnormal 

 conditions they may awaken into troublesome, and even 

 dangerous, activity. It is possible that many of these act 

 by producing unorganized ferments, and that the distinction 

 between the two kinds of ferments is rather superficial. 



It is now necessary to consider in detail the nature of 

 the various juices yielded by the digestive glands, and the 

 mechanism of their secretion, so far as it is known to us. 

 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 duty 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 upon its own basis ; but it must not be forgotten that 

 in the body digestion is the joint result of the chemical 

 rork of five or six secretions, the greater number of which 

 ire actually mixed together in the alimentary canal, and of 

 the mechanical work of the gastro-intestinal walls. 



The Chemistry of the Digestive Juices. 



(i) Saliva. The saliva of the mouth is a mixture of the 

 secretions of three large glands on each side, and of many 

 >mall ones. The large glands are the parotid, which opens 

 )y 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 mucous, are distinguished by structural characters and 

 by the nature of their secretion ; and the distinction has 



