GASTBIC DIGESTION. 337 



VIII. GASTEIC DIGESTION. 



Nearly all the digestive acts so far considered are purely mechanical 

 and solely preparatory to digestion in the sense in which the term diges- 

 tion implies production of changes essential to absorption. The food 

 has been seized, carried to the mouth and appreciated by the sense of 

 taste, masticated and impregnated by saliva, swallowed, and in the case 

 of ruminant animals again returned to the mouth for further preparatory 

 change. When once, however, it enters the stomach — and it must be 

 remembered that in the strict sense of the word this terra only applies to 

 the fourth stomach of ruminants — it is subjected to more or less pro- 

 found chemical and physical changes, which are described as resulting 

 from the processes of gastric digestion or chymification. As has been 

 already seen, the organ in which these digestive changes are inaugurated 

 is not distinctly defined in all species of animals, its first appearance 

 being a mere swelling of the alimentary tube without any distinct line of 

 demarcation at either extremity. Such a rudimentary stomach is found 

 in all animals below the subkingdom of the articulates. In the mollusks 

 and articulates its separation from the intestinal tube and oesophagus 

 becomes more evident, while in insects the stomach, with its glandular 

 appendages, becomes an important organ of digestion. 



In the fish the stomach is separated from the intestine by a narrow 

 pyloric orifice, but still lies in the direction of the long axis of the body, 

 as is also the case in many reptiles, though in the higher members of 

 this family and in the bird it acquires a considerable degree of com- 

 plexity and tends now to occupy a position at right angles to the axis 

 of the bod3'. The highest degree of complexitj' of the stomach is 

 found in the ruminant herbivora, and indicates that the diversity and 

 complexity of organization of the gastric parts is governed by the 

 peculiar alimentary habits and needs of the different species of animals. 



Certain general properties and characteristics of gastric digestion 

 are common to all the higher mammals, while, again, certain special dis- 

 tinctive points are found in the nature of gastric digestion according as 

 the animals are carnivora, ruminant or non-ruminant herbivora, or 

 omnivora. These general characteristics will first be alluded to, and the 

 peculiarity of gastric digestion in carnivora, ruminants, and non-ruminant 

 herbivora will subsequently demand attention. 



In the first place, we must consider the mode of accumulation of 

 food in the stomach, the changes in shape, and the motions thereby 

 inaugurated in that organ. Then we must studj- the secretions poured 

 out in the stomach as the result of contact of food with its Walls, their 

 composition and properties, and mode of separation from the blood ; 

 then the changes which the food undergoes in the stomach and during its 



23 



