64 The Vertebrate Organ Systems 



As blood is returned to the heart, it passes through the sinus 

 venosus into the right auricle. From the right auricle it passes into 

 the ventricle and out through the truncus arteriosus. The contractions 

 of the ventricle and truncus force the blood through the pulmo-cutaneous 

 artery to the skin and lungs where it is aerated. This blood is re- 

 turned from the lungs via the pulmonary vein to the left auricle and from 

 the skin by the cutaneous vein to the sinus venosus ; the blood from the 

 lungs enters the ventricle, but this time it is pumped through the truncus 

 arteriosus into the arteries leading to the main part of the body. There 

 are thus two chief routes of circulation : one to and from the skin and 

 lungs, the other to and from the body. The first is known as the pulmo- 

 cutaneous circulation, the second, as the systemic. 



A short distance from the point at which it leaves the heart, the 

 truncus arteriosus splits into two main branches. Each of these 

 branches almost immediately divides into three separate aortic arches: 

 a posterior branch, the pulmo-cutaneous artery, leading to the skin and 

 lungs ; a middle one, the systemic or aortic, leading to the legs and trunk ; 

 and an anterior one, the carotid, leading to the head. 



Since the blood from both paths of circulation must pass through 

 the ventricle and the truncus arteriosus, it would at first appear that 

 a great deal of mixing of oxygenated and unoxygenated blood must 

 occur. This is prevented to a great extent by some specialized struc- 

 tures, including the pitted surface of the ventricle and the spiral 

 valve of the truncus arteriosus. Both auricles contract simultaneously 

 to pump their contents into the ventricle. The blood remains but a 

 short time in this chamber, so a minimum amount of mixing occurs. 

 The nature of the wall also may help to prevent mixing. The unoxy- 

 genated blood from the right auricle is nearest the exit of the truncus 

 arteriosus and thus is the first to leave. It freely passes along the 

 surface of the spiral valve into the first available opening, the entrance 

 to the pulmo-cutaneous arch. When this arch is filled, the truncus 

 arteriosus contracts. This brings its ventral wall into contact with 

 the spiral valve and seals off the pulmo-cutaneous arch. The blood which 

 is now leaving the ventricle is from the left auricle and is aerated. It 

 is forced into the aortic and carotid arches. 



The Arterial System. — Arteries are the heavy-walled blood vessels 

 which carry the blood away from the heart. Usually the blood carried 

 by the arteries has a heavy load of oxygen. There is but one exception 



