RESPIRATORY CIRCULATION IN INSECTS. 273 



of the system.* The blood is then reconveyed to 

 the heart by the ordhiary veins, which form a large 

 vena cava (c). This vein is generally consider- 

 ably dilated at its termination, or just before it 

 opens into the auricle, constituting what has 

 been termed a venous sinus (s). This, then, is 

 followed by the auricle (d) and the ventricle (e) ; 

 but, besides these cavities, there is also a fourth 

 (f), formed by a dilatation of the beginning of 

 the branchial artery, and termed the hulbus arte- 

 riosus, contributing, doubtless, to augment the 

 impetus with which the blood is sent into the 

 branchial arteries. 



The circulation in Reptiles is not double, like 

 that of fishes ; for only a part of the blood 

 is brought under the influence of the air in 

 the pulmonary organs. All the animals belong- 

 ing to this class are cold-blooded, sluggish, and 

 inert; they subsist upon a scanty allowance of 

 food, and are astonishingly tenacious of life. 

 The simplest form in which we meet with this 

 mode of circulation is in the Batrachia; it is 



* The caudal branch of the aorta is protected by the roots of 

 the inferior spinous processes, joining to form arches through which 

 it passes ; and frequently the artery is contained in a bony chan- 

 nel, formed by the bodies of the vertebrae, which effectually se- 

 cures it from all external pressure. In the sturgeon even the ab- 

 dominal aorta is thus protected, being- entirely concealed within 

 this bony canal. 



VOL. II. T 



