432 



MANUAL OF ZOOLOGY. 



between the ventricles a perfect one, and even in these, as in 

 all other Reptiles, the heart consists functionally of no more 

 than three chambers. The ordinary course of the circulation, 

 where the ventricular septum is imperfect, is as follows : — The* 



impure venous blood returned 

 from the body is, of course, 

 poured by the vense cavae into the 

 right auricle (fig. 163, a), and 

 thence into the ventricle. The 

 pure arterialised and aerated 

 blood that has passed through 

 the lungs, is, equally of course, 

 poured into the left auricle {a'), 

 and thence propelled into the 

 ventricle (w). As the ventricular 

 cavity is single, and not divided 

 by a complete partition, it fol- 

 lows of necessity that there is a 

 mixture in the ventricle, resulting 

 in the production of a mixed 

 fluid, consisting partly of venous 

 and partly of arterial blood. 

 This mixed fluid, then, occupies 

 the common ventricular cavity, 

 and by this it is driven both to 

 the lungs (through the pulmonary 

 artery), and to the body (through 

 the systemic aorta). Conse- 

 ^ quently, in Reptiles, both the 

 . lungs and the various tissues and 



Fig. 163. — Diagram of the circulation in 

 Reptiles. (The venous system is left 

 light, the arterial system is black, and 

 the vessels containing mixed blood are 



f,^"vf„'o'utVio:d''J?o'ntb=idT:t; organs °f .the body are supplied 



Left auricle, receiving arterial blood With a miXtUre of arterial and 



from the lungs : z* Arterio-venous ven- „-,„„.,„ ui ] _J ._ -.i 



tricle, containing mixed blood, which is VCnOUS DlOOQ, and nOt With Un- 



driven by W the pulmonary artery to mixed blood the lunffS with 



the lungs, and by (0) the aorta to the , , , , °, 



body. purely venous, and the body with 



purely arterial blood — as is the 

 case with the higher Vertebrata. In the Crocodilia, as before 

 said, the partition between the ventricles is a complete one, 

 and consequently this mixture of the arterial and venous blood 

 cannot take place within the heart itself. In these Reptiles, 

 however, a direct comi^iunication exists between the pulmonary 

 artery and aorta (the right and left aort») by the so-called 

 "foramen Panizz»," close to the point where these vessels 

 spring respectively from the right and left ventricle. In these 

 Reptiles, therefore, the same mixture of arterial blood with 



