WARM-BLOODED CIRCULATION. 



199 



In the Saurian reptiles, the structure becomes again more 

 complicated. In the Chameleon each auricle of the heart 

 has a large venous sinus, appearing like two supplementary 

 auricles.'* The heart of the Crocodile has not only two au- 

 ricles, but its ventricle is divided by two partitions, into 

 three chambers: each of the partitions is perforated to allow 

 of a free communication between the chambers; and the pas- 

 sages are so adjusted as to determine the current of aerated 

 blood, returning from the lungs, into those arteries, more 

 especially, which supply the head and the muscles of the 

 limbs; while the vitiated blood is made again to circulate 

 through the arteries of the viscera.t 



It is in warm-blooded animals that the two offices of the 

 circulation are most efficiently performed; for the whole of 

 the blood passes, alternately, through the greater and the 

 lesser circulations, and a complete apparatus is provided for 

 each. There are, in fact, two hearts, the one on the left side 

 impelling the blood through the greater or systemic circu- 

 lation ; the other, on the right side, appropriated to the lesser, 

 or pulmonary circulation. The annexed diagram, (Fig. 35,9,) 

 359 .,-.^<:^ H illustrates the plan of the cir- 



^^ culation in warm-blooded ani- 



mals. From the left ventri- 

 cle (l) the blood is propelled 

 into the aorta (a,) to be dif- 

 fused through the arteries of 

 the system (b) to every part, 

 and penetrating into all the 

 capillary vessels; thence it is 

 returned by the veins, through 

 the venae cavae (c,)to the right 

 auricle (d,) which delivers it 

 into the right ventricle (e.) 

 This right ventricle impels 

 the blood, thus received, through the pulmonary arteries 



* Houston; Trans. Roy. Irish Acad, xv. 189. 



f It would appear, from this arrangement of the vessels, that the brain, or 



