186 



THE ANATOMY OF VERTEBEATED ANIMALS. 



whicli the carotid and systemic passages commence, while 

 that on the left side similarly leads to the entrance to the 

 pulmo-ciitaneous passages ; fourthly, that the carotid gland, 

 in which the carotid passage ends, presents a mechanical ob- 

 stacle to the flow of the blood through it ; fifthly, that there 

 is a valvular fold open towards the heart, in each systemic 

 passage, which also offers a certain amount of mechanical 

 resistance to the blood ; and, sixthly, that after the blood has 

 begun to flow through the bulb, it will gradually force the 



Fipr. 58. 



Fig. 58.— The Axolotl (Slredon). 



septum over to the left side, and so impede the flow into the 

 pulmo-cutaneous passage. 



Thus, when the auricular systole takes place, the right 

 auricle sends its venous blood into that division of the ventri- 

 cular cavity which lies nearest the opening of the bulb ; and, 

 when the ventricle contracts, the blood first driven into the 

 bulb is whoUy venous. This blood fills the passages on both 

 sides of the septum, but finds a veiy much greater resistance 

 to its exit on the right than on the left side. It therefore 

 flows, at first, exclusively into the left division, and makes 

 its way through the short pulmonary arteries into the lungs. 



