DIGESTIVE SYSTEM 745 



be an expansion on one side, or, as in pigeons, it may consist of median 

 as well as a pair of lateral chambers. The crop may be a simple res- 

 ervoir for food, or it may be a real glandular organ where secretions 

 are poured forth and digestion started. In fact, during the breeding 

 season pigeons secrete a milky fluid here which is used in feeding the 

 young. 



THE STOMACH 



The various portions of the stomach have already been named and 

 described in the frog. To the terms there given, should be added the 

 small curvature at the top or anterior surface of the stomach, usually 

 called the "lesser" curvature, and the posterior curvature called the 

 "greater." 



In some forms of animals such as amphibians, the lining of the 

 mouth, oesophagus, and stomach is covered with cilia. In birds the 

 stomach is divided into an anterior glandular region called the proven- 

 triculus, and a posterior muscular region called the gizzard. After the 

 food has passed through the proventricular region and has mixed with 

 the secretion from its glands it passes into the gizzard. This latter 

 organ is not only muscular but the muscles have developed into a pair 

 of disks with tendinous centers. There is a secretion in the gizzard 

 which hardens the lining sometimes, even raising little elevations which 

 are used in grinding the food. One might almost consider them teeth. 

 Remembering that birds have no true teeth, one can readily understand 

 the advantage such an animal has in a gizzard of this type. Grain-eating 

 birds swallow small pebbles which enter the gizzard and are thus also 

 made use of for grinding purposes. 



In fact, in the fossil pterodactyl there have been pebbles "found in 

 such portions as to lead to the supposition that these reptiles also had 

 a gizzard." It is well to note in this regard that the grain-eating birds 

 have the best developed gizzards, while birds of prey have gizzards much 

 less fully developed. In one species of pigeons a part of the wall of the 

 gizzard is ossified. In mammals there are more varying forms of stom- 

 achs. These are divided in from one to four regions. The ruminants 

 have two well developed divisions of the stomach (Fig. 435), the rumen 

 or paunch, and the reticulum or honey comb, though these two divi- 

 sions are really enlarged portions of the oesophagus and serve as res- 

 ervoirs of food. The food is regurgitated into the mouth for mastication, 

 and after it is swallowed a second time passes into the true stomach, the 

 psalterium (also called omasus or manyplies), and then to the abomasum 

 or rennet. The latter is used for gastric digestion. 



It is of interest here to trace the embryonic changes of the mesentery 

 in mammals. The mesentery supporting the stomach is called the meso- 

 gastrium. The first curvature of the stomach, which is toward the left, 

 broadens the corresponding part of the mesogastrium, an effect which is 



