212 ESSENTIALS OF ZOOLOGY 



base of the ventral aorta. When the auricles contract, there- 

 fore, and the venous and arterial blood enter the ventricle, the 

 latter immediately contracts and the spongy walls prevent 

 mixing, at all events on the right and left sides. The blood 

 of the right side of the ventricle is venous, that of the left 

 side is arterial, and the intervening blood is more or less 

 mixed. In the relaxed condition the longitudinal valve of 

 the conus lies over towards the right side. As soon, therefore, 

 as the blood rushes into the ventricle from the auricles the 

 venous blood is directed into the conus, and the pulsation of 

 the ventricle completely fills the left side of the conus. The 

 valve guarding the pulmo- cutaneous is overcome, and the 

 blood enters that vessel. The contraction of the ventricle 

 is immediately succeeded by that of the conus, and this 

 impels the longitudinal valve to the left, thus shutting off the 

 pulmo-cutaneous, and the more mixed blood, followed by the 

 arterial blood, forces open the semilunar valve and gains the 

 ventral aorta. In the ventral aorta the passage into the 

 systemic vessels is the wider, and the mixed blood enters on 

 each side until the pressure of the carotid, aided by the 

 carotid gland, is overcome. The arterial blood is thus received 

 by the vessels supplying the head. 



The blood in all the different regions of the body is spread 

 into smaller and smaller arteries and into capillaries. These 

 reunite to form veins. The principal veins may now be 

 referred to. 



The pulmonary veins run along the inner aspect of the 

 lungs and join above the heart to enter the left auricle by a 

 single opening. 



The blood from the head and anterior part of the body is 

 returned to a pair of anterior caval vessels by three main 

 vessels on each side : (1) the lingual and the mandibular 

 join to form the external jugular ; (2) the internal jugular 

 and the subscapular form the innominate ; (3) the brachial 

 and the rnusculo-cutaneous join to form the subclavian. 

 These three vessels run together to form the anterior vena 

 cava on each side. 



The posterior vena cava originates in the renal veins, 

 receives veins from the reproductive organs, passes through 



