284 



COMPARATIVE PHYSIOLOGY. 



The mouth parts, even iu some of the higher vertebrates, as 

 the Camivora, serve a prehensile rather than a digestive pur- 

 pose. This is well seen ia the dog, that bolts his food ; but 

 in this and allied groups of mammals gastric digestion is very 

 active. 



The teeth as triturating organs find their highest develop- 

 ment in ruminants, the combined side-to-side and forward-and- 

 backward motion of the jaws rendering them very efEective. 



In Camivora the teeth serve for grasping and tearing, while 

 in the Insectivora the tongue, as also in certain birds (wood- 

 peckers), is an important organ for securing food. 



Fig. S33.— Profile of upper teeth of the horse, more especially intended to show the 

 molars, the fangs haviiig been exposed (Chauveau). a, molar teeth; b, supple- 

 mentary molar; o, tusk; d, incisors. 



It is to be noted, too, that, wjjiile the horse crops grass by 

 biting it ofp, the ox uses the tongue, as well as the teeth and 

 lips, to secure the mouthful. 



r Man's teeth are somewhat intermediate in form between the 

 I carnivorous and the herbivorous type. Birds lack teeth, but 

 the strong muscular gizzard suffices to grind the food against 

 the small pebbles that are habitually swallowed. 



The crop, well developed in granivorous birds, is a dilatation 

 of the oesophagus, serving to store and soften the food. 

 ' > In the pigeon a glandular epithelium in the crop secretes a 



Fig. 234.— General view of digestive apparatus of fowl (after Chauveau). 1, tongue; 

 2. pharynx; 3, first portion of oesophagus; 4, crop; 5, second portion of (Bsopha- 



fus; 6, succentric ventricle (proventricnlus); 7, gizzard; 8, origm. of duodenum; 9, 

 rst branch of duodenal flexure; 10, second branch of same; 11, origin of floating 

 portion of small intestine; 12, small intestine; 12', terminal portion of this intes- 

 tine, flanked on each side by the two caeca (regarded as the analogue of colon of 

 mammals); 13. 13, free extremities of csecums; 14, insertion of these two euls-de- 

 sac into intestinal tube; 15, rectum; 16, cloaca; 17, anus; 18, mesentery; 19. left 

 lobe of liver; 20, right lobe; 21, gall-bladder; 22, insertion of pancreatic and biliary 

 ducts; the two pancreatic ducts are the most anterior, the choledic or hepatic is in 

 the middle, and the cystic duct is posterior; 23, pancreas; 24, diaphragmatic aspect 

 of lung; 25, ovary (in a state of atrophy); 26, oviduct. 



