THE RESPIRATORY ORGANS OF AMPHIBIA. 161 



in eacli 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 gradualjy 



Fig. 58.— The Axolotl {Siredon). 



force the 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 ventric- 

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

 when the ventricle contracts, the blood first driven into the 

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

 sides of the septum, but finds a very 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. 

 But, as the pulmonary vessels fill, the pressure on the two 

 sides of the septum becomes equalized, and the systemic pas- 

 sages, which offer the next least resistance, fill with blood, 

 which is now mixed, as it comes from the middle of the ven- 

 tricle. Next, the septum, being driven over to the left side, 

 prevents any more blood from going into the pulmo-cutane- 

 ous passage. At the end of the systole, the blood driven out 

 by the ventricle is almost wholly that of the left auricle ; and, 

 by this time, the resistance in the systemic is as great as that 

 in the carotid passages. Hence the latter fill, and send arteri- 

 alized blood to the head. 



The organs of respiration of the Amphibia, in the adull 



