200 CHARACTERS OF VERTEBRATE ANIMALS 



coiled small intestine, and short wide large intestine, terminating 

 in a cloaca. A large liver, provided with a gall-bladder, and a 

 small pancreas, open into the beginning of the small intestine. 



Circulatory Organs (fig. 131). — As in Mammals and Birds, we 

 can distinguish between blood system and lymph system, of which 

 only the former need here be considered. 



Blood System. — This is a closed system of tubes, consisting of 

 heart, arteries, veins, and capillaries, in which the cold blood 

 circulates. Seen under the microscope, a drop of lizard's blood 



POSTCAVAL BRIGHT 



AURICLE 



Fig. 131. — General Structure of Lizard, cer.h., cerebral hemisphere 



presents much the same appearance as a drop of bird's blood, 

 and is made up of a clear liquid plasma, in which float irregular 

 white corpuscles capable of executing creeping movements, and 

 oval red corpuscles, each of which is a nucleated cell. 



The heart is enclosed in a double-walled pericardial sac, and 

 is distinctly a more imperfect organ than in the Mammal or Bird, 

 as, though there are two auricles, there is only one ventricle, and 

 therefore the impure blood received by the right auricle from the 

 body mixes to some extent with the pure blood received from the 

 lungs by the left auricle. As will be seen, however, there are 

 several devices by means of which the mixture of the two sorts 

 is partly prevented. 



The three great caval veins, two in front and one behind, bring 

 back the impure blood to the heart, as in a Mammal or Bird (see 

 fig. 102), but, instead of opening directly into the right auricle, 

 they pour their blood into a thin-walled pouch, the venous sinus, 

 which communicates by a valvular opening with the right auricle. 

 As usual, the purified blood from the lungs is poured by pulmonary 

 veins into the left auricle. The two auricles squeeze their blood 

 through a single valvular opening into the thick-walled ventricle. 



