SECTION II. 

 DIGESTION. 



I. GENERAL CHARACTERISTICS OF THE DIGESTIVE APPARATUS. 



DIGESTION is the preparation of food for absorption, and is usually 

 accomplished by the introduction of the food into a special cavity com- 

 municating with the exterior, where it undergoes such changes as will 

 enable it to pass through the walls of the blood-vessels. Foods of 

 animals, as alreacty shown, are usually solids ; that they may be ab- 

 sorbed and enter into the blood of animals they must first be reduced 

 to a fluid condition; this solution is the object of digestion and is 

 accomplished by means of the different secretions poured out by the 

 alimentary canal. It is evident, therefore, that the alimentary tract must 

 consist of a cavity to contain these digestive fluids ; must communicate 

 with the exterior to permit of the entrance of food and removal of 

 indigestible residue ; that it must be provided with motor organs for 

 determining the entrance of food, and that its walls must be capable 

 of elaborating digestive secretions and absorbing the results of the 

 digestive process. Digestion, therefore, includes a number of complex 

 processes : the prehension of food and in many cases its mechanical 

 comminution or mastication by special organs; secretion, or the mode 

 of production of the digestive fluids ; absorption, or the means of con- 

 veyance of the digestive products into the blood stream; and finally 

 defecation, or the expulsion of the non-nutritious residue. If the 

 digestive tract is considered from the point of view of these different 

 purposes, it will be found to be very differently constituted in different 

 members of the animal kingdom, its state of development governing the 

 complexity of all accessory organs. 



In the higher animals it consists of the mouth, the pharynx, gullet, 

 stomach, intestine, and anal aperture. In its development it is found 

 to be simply a continuation of the external surface reflected inward as 

 we would turn in the finger of a glove. Consequently, in the gradual 

 evolution of the alimentar}' canal from its simplest to its most com- 

 plex form, we find every grade of such reflection, from a mere depression 

 in the external surface, as in the amoeba, to the long and complicated 

 intestinal tube of the ruminant. In all cases this is the mode of origin 

 of the alimentary canal ; consequently, food, when within the alimentary 



(203) 



