122 VERTEBRATA. 



the sympathetic. This is formed by special branches of the spinal 

 nerves and spinal-like cranial nerves, which are connected with special 

 ganglia and give off nervous plexuses to the viscera (fig. 80). 



The organs of nourishment, circulation, and reproduction are 

 placed in the body cavity which extends beneath (ventral to) the 

 skeletal axis. The digestive canal is a more or less elongated tube 

 which in the region of the skull is encircled by the visceral arches ; it 

 begins with the mouth and ends Avith the anus, which latter is placed 

 on the ventral surface at various distances from the hinder end of 

 the body (according to the length of the caudal region of the verte- 

 bral column). The alimentary canal is invested in the greater part 

 of its course by a fold of the peritoneum which lines the body cavity, 

 and is fastened to the under surface of the vertebral column by the 

 two lamellse of this fold, which are closely applied to one another 

 and form the mesentery. As a rule the alimentary canal is much 

 longer than the distance between the mouth and anus, and therefore 

 forms more or less numerous coils in the body cavity. 



The digestive canal is almost always divided into three regions, the 

 oesophagus and stomach, the small intestine with liver and pancreas, 

 and the large intestine. The oesophagus always begins with a buccal 

 cavity, on the floor of which a muscular fold, the tongue, projects. 

 Although this organ, which is richly supplied with nerves, is in 

 general rightly regarded as an organ of taste, it nevertheless plays a 

 considerable part in the reception of the food, and may even in some 

 cases altogether lose its importance as an organ of taste. The buccal 

 cavity, except in Amphioxus and the Cyclostomes, is enclosed by the 

 skeletal arch known as the maxillo-palatine apparatus and the lower 

 jaw, of which the latter is always capable of powerful movements, 

 while the parts of the former are either more or less firmly united 

 together and attached to the bones of the skull, or are capable of 

 movement on the latter. The two jaws, unlike those of the Arthro- 

 poda, work upon one another in the direction from below upwards. 

 They are usually furnished with teeth. The teeth are derived from 

 ossified papillae (dentine) of the mucous membrane of the mouth 

 (fig. 579), which are covered with an epidermal structure the enamel; 

 they are either directly fused with the bones of the jaw or inserted 

 into special alveoli in the latter. The teeth in the higher Vertebrates 

 are confined to the upper and lower jaws, but in the lower Vertebrates 

 they may appear on all the bones which surround the buccal cavity. 

 Teeth are, however, often altogether absent. In Birds and Tortoises 

 they are replaced by a horny covering of the sharp edges of the jaws 



