CHAKACTEKISTICS OF THE DIGESTIVE APPAEATUS. 213 



and which has for its function the crushing and mastication of food. The 

 proventriculus, ventriculus succenturiatus, or true glandular stomach, 

 varies in form and size in different birds, being sometimes wide and 

 straight and sometimes round. In the rasorial birds it is wider than the 

 gullet and smaller than the gizzard. Its mucous membrane is thicker 

 than that of the cesophagus, and furnished with tubular glands which 

 secrete an acid digestive secretion. In the grain-eaters these glands are 

 sacculated, or expanded into compound follicles, the disposition of the 

 glands varying in different species. The gizzard, ventriculus bulbosus, the 

 third or muscular stomach, is a more or less flattened, ovoid organ, hav- 

 ing two apertures at its upper part, one communicating with the proven- 

 triculus, the other with the small intestine. The gizzard is feebly devel- 

 oped, or may be even absent in carnivorous birds, such as the crow and 

 the raven, and is there simply a membranous expansion of the stomach, 

 free from secreting membrane, and bearing close analogy and function 

 with the membranous cardiac extremity of the stomach of the horse. 

 The intestines of birds are, as a rule, relatively to the size of the body, 

 shorter than those of mammalia, but longer than those of reptiles. In 

 birds of prey, as a rule, they are not more than twice as long as the body, 

 including the bill, but in the osprey they are eight times as long. In 

 fructivorous and granivorous birds they are much longer. The duodenum 

 forms a loop, embracing the pancreas. The division between small and 

 large intestines is not clearly marked, as villi are found in both. The 

 point of entrance of the caeca, which are most developed in birds feed- 

 ing on vegetable food, marks the union of small and large intestines. 



In all the groups of animals already referred to the stomach occupies 

 a position in the long axis of the body. It is only in mammals that its 

 position becomes transverse (Fig. 72), and we notice that even in these 

 animals this transverse position becomes more accentuated during its 

 state of functional activity. Thus, when fasting the pyloric orifice of 

 the stomach sinks and the organ tends to assume a longitudinal position ; 

 when filled with food it undergoes a partial rotation on its own axis, the 

 pyloric orifice ascends, and it now becomes transverse. 



In mammals the cesophagus is only destined to convey food to the 

 stomach ; it has contractile walls, but few or no glands, and the pouches 

 which we have recognized in the birds are represented in but a single 

 group of mammals, — the ruminants, — and here they are situated so low 

 down in the cesophagus as to be ordinarily described as divisions of the 

 stomach. Their function and structure prove that they maybe regarded, 

 nevertheless, as oesophageal pouches. The diameter of the cesophagus 

 varies according to the food which serves as the normal diet for these 

 animals. It is large and readily dilatable in carnivora, which bolt their 

 food entire ; it is narrow in the herbivora ; and in those animals which 



