116 OF THE BLOOD 



ventricle, the blood is driven through the main arterial trunk, 

 called the aorta, (Fig. 90, a,) and is distributed by its 

 branches throughout the body ; it is then collected by the 

 veins, carried back to the heart, and poured into the right 

 auricle, (Fig. 85, r a,} which sends it into the right ventricle 

 rv.) The right ventricle propels it through another set of 

 arteries, the pulmonary arteries, (Fig. 90, p,) to the lungs, 

 (Z ; ) it is there collected by the pulmonary veins, and con- 

 veyed to the left auricle, (Fig. 85, I a,] by which it is returned 

 to the left ventricle, thus completing the circuit. 



241. Hence the blood in performing its whole circuit 

 passes twice through the heart. The first part of this cir- 

 cuit, 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 cir- 

 culation : this double circuit is said to be a complete circu- 

 lation. In this case the heart may be justly regarded as 

 two hearts conjoined, and in fact the whole of the lesser cir- 

 culation intervenes in the passage of the blood from one side 

 of the heart to the other ; except that during the embryonic 

 period there is an opening between the two auricles, which 

 closes as soon as respiration commences. 



242. In reptiles, (Fig. 86,) 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 sep- 

 arate the two kinds of blood received from the auricles ; but 

 the mixture soon takes place by means of a special artery, 

 which passes from the pulmonary artery to the aorta. 



243. In fishes, (Fig. 87,) the blood is carried directly 



