THE PURPOSE OF DIGESTION 227 



3. The purpose and means of Digestion.— All food- 

 stuffs being thus proteins, fats, carbohydrates, or mineral 

 matters, pure or mixed up with other substances, the 

 whole purpose of the alimentary apparatus is in the first 

 place to separate these proteins, &c., from the in- 

 nutritious residue, if there be any, and to reduce them 

 to a condition of fine subdivision and ultimately to 

 one of solution, in order that they may make their way 

 through the delicate structures which form the walls of 

 the vessels of the alimentary canal. In the next place 

 this mechanical and physical change must be accompanied 

 by chemical changes whereby the food-stuffs are brought 

 into such a condition that when they reach the tissues 

 the latter can take them up or assimilate them. 



To these ends food is taken into the mouth and 

 masticated, is mixed with saliva, is swallowed, undergoes 

 gastric digestion, passes into the intestine, and is sub- 

 jected to the action of the secretions of the liver and 

 pancreas with which it there becomes mixed ; and, finally, 

 after the more or less complete extraction of the nutritive 

 constituents, the residue, mixed up with certain secretions 

 of the intestines, leaves the body as the faeces. 



The actual digestive changes of food are brought about 

 chiefly by the action of fluids secreted by glands whose 

 ducts pour their secretions into the cavity of the ali- 

 mentary canal. These glands are essentially groups of 

 cells supplied with nerves and blood-vessels ; but the 

 arrangement of these cells may be simple or complicated, 

 as in the types shown in Fig. 68. 



Thus the glands of the walls of the intestines are tubular 

 (1). Those in the walls of the stomach are also tubular 

 but divided into two or more parts at their inner end 

 (2). The salivary glands and the pancreas are more com- 

 plicated ; their ducts divide and subdivide into multitudes 

 of smaller tubes, each of which ends in a dilatation in 

 which the secreting cells lie (6). These dilatations, at- 

 tached to the branched ducts, somewhat resemble a 



Q 2 



