202 OF THE BLOOD AXD CIBCTTLATION. 



conveyed to the left auricle (/, a), by which it is returned to 

 the left ventricle, thus completing the circuit. 



Sup. vena cava. Pul. art. Aorta (a). Pulmonary artery (p). 

 Pulmonary veins (p v). \ fy^~^\ ' ...Pulmonary veins (p ;). 







Right auricle (r a). /jji|^(-^flm Left auricle (Z a). 



, ^__^., , .^v,. ., Mitral valve. 

 Tncuspid valve. 



Inferior vena cava. 



Right ventricle (r i;). / \^fF/ Left ventricle (I ). 



[ 

 Partition. Aorta descending (a). 



Fig. 218. Ideal section of the human heart. 



365. Hence the blood, in performing its whole circuit, 

 passes twice through the heart. The first part of this circuit, 

 the passage of the blood through the body, is called the great 

 circulation, and the second part, the passage of the blood through 

 the lungs, is the lesser or pulmonary circulation : this double 

 circuit is said to be a complete circulation (fig. 217). In this 

 case, the heart may be justly regarded as two hearts conjoined, 

 and, in fact, the whole of the lesser circulation intervenes in the 

 passage of the blood from one side of the heart to the other ; 

 except that during the embryonic period, when there is an 

 opening between the two auricles, which closes as soon as 

 respiration commences. 



366. In reptiles (fig. 219) the venous blood from the 

 body is received into one auricle, and the oxygenated blood 

 from the lungs into the other. These throw their contents 

 into the single ventricle below, which propels the mixture in 

 part to the body, and in part to the lungs ; but as only the 

 smaller portion of the whole quantity is sent to the lungs in a 

 single circuit, the circulation is said to be incomplete. In the 

 crocodiles, the ventricle has a partition which keeps separate 

 the two kinds of blood received from the auricles ; but ,,he 



