VERTEBRATA : REPTILIA. 



529 



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 1 ), 

 and thence propelled into the 

 ventricle (?'). 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, result- 

 ing 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 

 organs of the body are supplied 

 with a mixture of arterial and 

 venous blood, and not with un- 

 mixed blood the lungs with 

 purely venous, and the body with 

 purely arterial blood as is the F; 

 case with the higher Vertebrata. 

 In the Crocodilia, as before said, 

 the partition between the ventri- 

 cles is a complete one, and con- 

 sequently this mixture of the 

 arterial and venous blood can- 

 not take place within the heart 

 itself. In these Reptiles, how- 

 ever, a direct communication exists between the pulmonary 

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

 "foramen Panizzae," 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 

 venous takes place as in the lower Reptilia^ though probably 

 not to so complete an extent. It is this peculiarity of the 

 circulation in all Reptiles which conditions their low tempera- 

 ture, slow respiration, and generally sluggish vital actions. 



The lungs in all Reptiles, except the Crocodiles, are less 

 completely cellular than in the Birds and Mammals, and they 



2 L 



Diagram of the circulation in 

 ptiles. (The venous system is left 

 light, the arterial system is black, and 

 the vessels containing mixed blood are 

 cross-shaded.) a Right auricle, receiv- 

 ing venous blood from the body; a' 

 Left auricle, receiving arterial blood 

 from the lungs ; v Arterio-venous ven- 

 tricle, containing mixed blood, which 

 is driven by (/) the pulmonary artery 

 to the lungs, and by (o) the aorta to 

 the body. 



