AMPHIBIA. 203 



lungs by the pulmonary veins, which open into the left auricle. 

 The caval veins pour impure blood from the general body into 

 the right auricle. 



The blood in the precavals is partly oxygenated, as they receive the 

 cutaneous veins from the skin, while that in the postcaval is largely free 

 from nitrogenous waste. 



The blood is pumped out of the heart by the contraction of its 

 walls. The sinus venosus first undergoes systole, then the auricles 

 together, then the ventricle, and lastly the truncus arteriosus. 

 Systole is followed by diastole, and the various valves prevent 

 regurgitation. 



Although the ventricle is single, and receives both kinds of 

 blood, yet these do not completely mix, and the carotid, systemic, 

 and pulmo-cutaneous arches are supplied by oxygenated, mixed, 

 and impure blood respectively. This is explained as follows : 

 The cavity of the ventricle is transversely elongated. Into its 

 left side the auricles open (their common aperture being rendered 

 practically double by the free edge of the auricular septum), and 

 out of its right side the truncus arteriosus. Moreover, its spongy 

 wall absorbs much of the blood received by the auricular systole, 

 and renders mixing more difficult. Just previous to the ventri- 

 cular systole, the three arches offer different degrees of resistance 

 to the passage of blood the pulmo-cutaneous least, owing to 

 the small extent of their branches, the carotid most, on account of 

 the spongy carotid glands. When the ventricle contracts, the 

 blood nearest the truncus is impure, as the right auricle fills the 

 right side of the ventricle. This blood takes the direction of 

 least resistance i.e., runs on the left of the longitudinal septum 

 of the truncus into the pulmo-cutaneous arches. As these are 

 filled, their resistance increases, and blood (now mixed, as 

 sufficient time has elapsed for mixture to commence in the ven- 

 tricle) flows on the right side of the septum, flapping it over the 

 pulmo-cutaneous aperture, and enters the systemic trunks. Their 

 resistance also increases, and the last portion of blood runs into 

 the carotid arches. This is oxygenated blood from the left 

 auricle, which has been stored up in the spongy wall of the left 

 side of the ventricle. 



The large amount of elastic and muscular tissue in the arterial 

 walls serves two useful purposes. (1) Part of the force of the 

 systole of the ventricle and truncus is expended in dilating the 



