XI HEART OF AMPHIBIA 415 



blood. This first blood to pass into the conus will clearly be that which 

 was in the neighbourhood of its ventricular opening — ^the blood which 

 was in the right part of the ventricle, i.e. venous blood. Of the three 

 pairs of aortic arches the blood first passes into that in which the resistance 

 is least — the sixth pair of arches leading to the comparatively short 

 and simple circuit of vessels in the lungs. The lungs therefore receive 

 venous blood. 



The wall of the conus now itself contracts ; it is forced against the 

 edge of the spiral septum and as a result the pulmonary cavity is isolated 

 and the blood coming into the conus has now only two possible routes 

 before it. Of these it again chooses that with the lowest resistance 

 through the fourth (systemic) arches which lead to the greater part of 

 the body. The third arches which form the roots of the carotid arteries 

 offer the highest degree of resistance to the passage of blood, b' ..g 

 provided with special blocking valves in their interior to ensure this. 

 The result is that it is only the last of the blood from the ventricle — 

 the arterial blood from its left side — that, after the resistance in the 

 fourth arches reaches its maximum through their being filled with blood, 

 passes into the carotid vessels and so to the brain. It follows then that 

 just as the blood poorest in oxygen is sent to the lungs so that richest 

 in oxygen is sent to the brain. 



Comparing the heart of the frog with that of the lung-fish (Lepidosiren) 

 it is to be noticed that it shows certain features which are found in the 

 heart of the more highly developed tetrapods. (i) It has assumed the 

 general form characteristic of the higher vertebrates. (2) The atrial 

 septum is complete. (3) The conus arteriosus has become greatly 

 shortened so as to straighten out the double fold present in the lung- 

 fish and convert it into a regular spiral twist (as shown by the spiral 

 course of the longitudinal ridge in its interior). 



On the other hand the heart of the frog is further removed from that 

 of the higher vertebrates than is that of Lepidosiren in the fact that the 

 ventricle is undivided. Correlated with this is the quite peculiar method 

 of directing the streams of arterial and venous blood to their respective 

 destinations. 



In the Amphibia in general the heart is constructed on the same plan 

 as that of the frog — with various differences in detail. In the Urodela 

 and Apoda the atrial septum is incomplete and in the same groups there 

 is a well-marked tendency to reduction of the spiral septum of the conus. 

 In the newts (Triton) it is found in some cases to have reverted to the 

 condition of a longitudinal row of separate elements while in others it 



