IV. VERTEBRATA: MAMMALIA. 627 



many insectivores and rodents) and, on the other hand, beneath the molars 

 the anlagen of replacing teeth may be found. The latter fact shows 

 that the molars, strictly speaking, belong not to the permanent but to the 

 milk dentition. They are late in formation and are therefore parts of the 

 first dentition carried over into the second. 



The mouth, which contains tongue and teeth, is separated from 

 the next division of the alimentary tract, the pharynx, by the 

 uvula. The pharynx narrows behind into the oesophagus, the en- 

 trance of which into the stomach is marked by a constricting 

 cardia. At its other end the stomach has a similar constrictor, 

 the pylorus, separating it from the intestine. In the latter small 

 and large intestines (the latter consisting of colon and rectum) 

 are differentiated by the diameter of the lumen. The small in- 

 testine opens laterally into the colon and at the junction arises a 

 blind diverticulum, the caecum, which is small in mammals with 

 animal food, but in herbivores (especially rodents) is always large 

 and forms a conspicuous part of the alimentary tract. The ver- 

 miform appendix (primates, rodents) is a narrower part of the 

 caecum. Three pairs of salivary glands empty into the mouth, 

 the liver and pancreas into the small intestine (duodenum). 



Most important of respiratory peculiarities is the diaphragm, 

 which separates the body cavity into thoracic and abdominal cavi- 

 ties. This occurs only in its beginnings in other vertebrates 

 (perhaps even in Amphibia). In the thoracic cavity are the 

 O3sophagus, heart with its pericardium, and especially the trachea, 

 bronchi, and lungs; the remaining vegetative organs are in the 

 abdominal cavity. The diaphragm is a muscular dome, its con- 

 vex side towards the thoracic cavity; by contraction it flattens an 

 increases the size of the cavity, in consequence of which air is 

 drawn into the lungs (inspiration). On relaxation the lungs con- 

 tract from their own elasticity and force out a part of the air 

 (expiration). The intercostal muscles, which raise and lower the 

 framework of the chest, also play a part, as in birds. The respira- 

 tory ducts (fig. 579) begin with the larynx (with vocal cords), 

 which can be closed from the pharynx by the epiglottis; this is 

 followed by the trachea, which divides into right and left bronchi. 

 Each bronchus divides again and again, and the finest of these 

 divisions, the bronchioles, are continued as alveolar ducts to small 

 chambers, the infundibula, both these and the alveolar ducts being 

 lined with small respiratory pockets, the alveoli. 



The heart, with two auricles and two ventricles, is completely 

 separated into systemic and pulmonary halves. In early embryonic 



