RESPIRATORY CIRCULATION IN INSECTS. 273 



of the system.* The blood is then reconveyed 

 to the heart by the ordinary' veins, which form a 

 large vena cava (c). This vein is generally con- 

 siderably dilated at its termination, or j list before 

 it opens into the auricle ; constituting what has 

 been termed a venous sinus. This, then, is fol- 

 lowed 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 bulbus 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 belonging 

 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 

 channel, formed by the bodies of the vertebrae, which effectually 

 secures it from all external pressure. In the Sturgeon even the 

 abdominal aorta is thus protected; being entirely concealed 

 within this bony canal. 



VOL. II. T 



