134 



VITAL FUNCTIONS. 



On inspecting the plan, Fig. 84, it may be remarked 

 that in these animals only one half of the blood is sent 

 to the lungs before it again circulates through the gene- 

 ral system. The general circulation, therefore, consists 

 of one-half arterial, and one half venous blood. We 

 shall see directly that in the warm blooded animals the 

 circulation is double, and that in these, all the blood is 

 sent through the lungs, to be aerated or exposed to the 

 influence of the atmosphere, before it is thrown into the 

 general circulation. It will be seen also, that the tem- 

 perature and vivacity of the latter class, depend on the 

 exposure of the whole mass of blood, to the influence 

 of the oxygen of the atmosphere, as it passes the lungs. 



It is owing chiefly to this limited circulation that 

 amphibious animals are so remarkably distinguished 

 from others. They are not only cold blooded, but most 

 of them are sluggish, languid, and exceedingly tenacious 

 of life, so that they will not only bear the strongest 

 stimulants without injury, but may have their limbs 

 amputated with only slight marks of pain. 



Circulation in Fishes. In Fishes the organs of circu- 

 lation consist of four cavities, c, 

 d, e,f, Fig. 85, with a system of 

 veins and arteries for conveying 

 the blood to, and from the heart. 

 Of these cavities, d, is the auricle, 

 fii and e, the ventricle, c, and f, 

 being dilations of the principal 

 vein and artery, at their junction 

 with the heart. The heart, in 

 this system, belongs exclusively 

 to the gills or branchia, which in 

 Fish are the organs of respira- 

 tion. There is no aorta procee- 

 ding from the heart, which car- 

 ries the blood to all parts of the 

 system, as in other animals. 

 The branchial arteries/, convey 



What kind of blood circulates through the systems of amphibious 

 animals 1 To what cause is the coldness and languor of these animals 

 owing ? Which is the auricle and which the ventricle in the heart of a 

 fish* 



Fig. 85. 



