58 MORPHOLOGY OF THE VERTEBRATE BODY 



lie dorsal and anterior to the single ventricle and open by a common 

 aperture provided with valves. The ventricle connects with an 

 outgoing vessel, the iruncus arteriosus, the opening to which is 

 again protected by valves leading in one direction. The truncus 

 arteriosus divides, right and left, into three large arteries, the 

 carotid, systemic, and pulmo-cutaneous arteries, passing to the head, 

 to the body, and to the lungs and skin, respectively. Within 

 the truncus is another valve, guarding the openings of the sys- 

 temics and carotids. This does not appear in the figure. If one 

 watches the contractions of the heart in a freshly killed frog, it is 

 seen that the two auricles contract simultaneously, then the ven- 

 tricle, and lastly the truncus. By gently lifting the ventricle it 

 will be seen that the simis venosus, which connects with the right 

 auricle and contains blood returned from all parts of the body 

 save the lungs, also contracts an instant before each contraction 

 of the two auricles. The " contraction " of the heart in the 

 frog is, therefore, a series of contractions, including not only the 

 auricles and ventricle, but also the sinus venosus and the truncus 

 arteriosus. Appropriate valves guard the several openings, and 

 a periodic repetition of the contractions results in circulation. 



From the structural and functional relationships, it would appear 

 that the separation of oxygenated and unoxygenated blood can 

 be effected in the following manner. When the two auricles con- 

 tract they force their contents simultaneously into the single ven- 

 tricle. The left auricle contains oxygenated blood from the lungs, 

 and the right auricle contains unoxygenated blood from the body. 

 By referring to Fig. 32, it will be seen that, roughly speaking, 

 the side of the ventricle on the observer's left is now occupied by 

 unoxygenated blood, and the side on his right by oxygenated 

 blood. If the two remained for even a brief time in the ventricle, 

 complete mixing would occur. But the ventricle contracts at once; 

 and, since the opening to the truncus is located on the side con- 

 taining the unoxygenated blood, it is this blood that leaves the 

 ventricle first of all. Blood from the middle region follows, 

 and, last of all, the blood from the side of the heart on the observer's 

 right enters the truncus. The mechanism of the valves within 

 the truncus and the conditions of pressure are such that the 

 unoxygenated blood that enters first flows at once into the 

 two pulmo-cutaneous arteries and thence reaches the lungs 

 and skin. Blood delivered to the truncus from the middle region 



