48 PHYSIOLOGY. 



scopical glands whose ducts open directly upon the lining, interior 

 surface. Outside the canal, their secretions emptying into the canal 

 by small ducts, are the larger glands, salivary, liver, and pancreas. 

 The ducts of the salivary glands open into the mouth; the common 

 duct of the liver and pancreas opens into the first fold of the small 

 intestine, the duodenum. 



Although digestion in its entirety, as it occurs in the alimentary 

 canal, is in its nature very complex, yet there are three natural divi- 

 sions of the whole process based upon the changes as they occur (1) 

 in the mouth (including the pharynx and oesophagus), (2) in the 

 stomach, and (3) in the intestines. 



It is the intention to consider the changes and alterations of the 

 foodstuffs, whether mechanical or chemical, in each, together with 

 the anatomy of the parts of each division and the structure of the 

 accessory glands, with their secretions and the functions they bear to 

 the completion of the entire work. However, the fact must not be 

 lost sight of that these divisions are only arbitrary and for conve- 

 nience, as no real line can be drawn at the various stages, since all 

 parts, structures, and functions work in harmony, on the plan of 

 division of labor: having in mind one common end the dissolving 

 of the food so that it can become a part of the circulation. 



PREHENSION. 



Before the processes of digestion can begin, it is essential that 

 the food should be brought to and placed in the mouth, the beginning 

 of the alimentary canal, for only in some of the infusoria does diges- 

 tion of the food take place outside the organism,, due to the influence 

 of ferments secreted by the organism to be nourished. The act of 

 bringing the food to the mouth has been termed prehension. 



Nature has admirable contrivances for this act wherever we look 

 among the lower animals. The monkey, squirrel, rat, etc., usually 

 make use of their anterior extremities for grasping and bringing to 

 their mouths the food, while they sit upon their haunches. The 

 horse makes use of his teeth and lips; indeed, his upper lip is very 

 movable, long, and endowed with extreme sensibility. It is his means 

 of gathering together his grain and bringing it to the incisors which 

 cut it up, then to be passed along by the tongue to the molars for 

 grinding. In the cow, the tongue, in the cat and dog, the teeth and 

 jaws, are the main organs of prehension. The frog, by protruding 

 his long, thin tongue, the surface of which is covered with a viscid 

 mucus, catches insects as they fly. 



