VERTEBRATA REPTILIA. 43 



to the skull by a quadrate bone. The various pieces, as 

 well as the two halves, are anchylosed in the tortoises. 

 In the snakes the two halves of the lower jaw are only 

 united by muscles and ligaments. 



44. Digestion. All reptiles, except the turtles and tor- 

 toises, possess teeth. The teeth in the various parts of the 

 jaw are all of the same kind, and, except in the crocodiles, 

 are not inserted in separate sockets. The tortoises and 

 turtles have horny beaks instead of teeth. Generally 

 speaking, there is not much peculiarity in the digestive 

 system of the Reptilia. Salivary glands are sometimes 

 absent. In serpents the stomach is but little different 

 from the gullet and intestine ; in crocodiles it bears a 

 considerable resemblance to the gizzard of birds. The 

 crocodiles also swallow pebbles to assist in grinding the 

 food. The numerous conical teeth with which these 

 animals are provided, are better adapted for holding the 

 food than for masticating it. The intestine, as in birds, 

 opens into a cloaca, which also receives the secretions of 

 the kidneys and genital organs. 



45. Circulation. In mammals and birds the heart 

 always contains four cavities two auricles and two ven- 

 tricles ; and the right and left sides are always distinct, the 

 chambers of the right side containing only venous blood, 

 and the left side arterial blood. In reptiles the heart has 

 always two auricles ; sometimes one, and sometimes two 

 ventricles. When there is only one ventricle, the follow- 

 ing is the course of the circulation : The impure venous 

 blood from the body is poured by the venae cavse into 

 the right auricle, thence into the ventricle ; the arterial 

 blood is conveyed by the pulmonary veins to the left 

 auricle, and passes also into the ventricle. This cavity 

 thus contains a mixture of venous and arterial blood. 

 This mixed fluid is driven by the ventricle, partly to the 

 lungs by the pulmonary artery, and partly by the aorta 

 to all parts of the body. 



In the crocodiles there is a complete partition between 

 the two ventricles, the heart in this group containing four 



