CH. VIII] FROGS, TOADS, AND NEWTS 259 



posterior to the auricles ; it receives blood from both 

 auricles ; (4) the truncus arteriosus, which leads out from 

 the right anterior ventral corner of the ventricle. 



The blood passes through the several chambers in the 

 above order, and is prevented from going in the contrary 

 direction, when the walls of the heart contract, by valves 

 placed at the auriculo- ventricular openings and on the 

 truncus arteriosus, and by the fact that at each beat of the 

 heart a wave of contraction sweeps continuously over the 

 whole set of chambers commencing at the sinus venosus 

 and ending at the truncus arteriosus. Thus the contrac- 

 tion itself urges the blood in the proper direction and 

 opposes any reflux. Since the heart continues to beat 

 for some time after general death the various phases and 

 appearances can be readily observed. 



The ventricle contains both venous and arterial blood, 

 venous on the right and arterial upon the left side, while 

 the two to a certain extent mingle in the centre. There 

 is however no doubt that much of the blood returned from 

 the skin and mouth into the right auricle is fairly rich in 

 oxygen (vide p. 261, Respiration). The valves within the 

 truncus arteriosus are so disposed that the most venous 

 blood is directed along the pulmo-cutaneous arteries to 

 the lungs and skin. The mixed blood is conveyed by the 

 systemic aortse to all the rest of the body except the head, 

 which receives the most arterial blood by the carotid 

 arteries. Thus the brain is supplied with better oxygen- 

 ated blood than any other part of the body. 



The two systemic aortse sweep round to the dorsal 

 side of the gullet, and unite posteriorly to form the 



172 



