48 A Manual of Veterinary Physiology. 



arterial or purified blood is brought back from the lungs 

 for distribution over the body. The former is often called 

 the Pulmonic, the latter the Systemic circulation. 



Mention has been made of valves in the cavities of the 

 heart ; we find them on both right and left sides separating 

 auricle from ventricle, known as the right auriculo-ventri- 

 cular or tricuspid valve, and the left auriculo-ventricular 

 or mitral valve. Besides these, valves are found in the 

 vessels arising from the ventricles, viz., in the pulmonary 

 artery and the aorta. These valves called pulmonary and 

 aortic are often spoken of as the semi-lunar valves. No 

 valves are found guarding the entrance of the vessels into 

 the auricles. 



In order that we may understand the function of these 

 valves, which play such an important part in the physiology 

 of the heart, it is necessary that we should briefly detail the 

 course which the blood takes from the time it enters the 

 right auricle until it completes the round of the circulation 

 and rinds itself at the auricle again. 



The venous blood flowing into the right auricle by means 

 of the anterior and posterior vena cava, passes from this 

 through the tricuspid valve into the right ventricle ; from 

 here it passes to the lungs by means of the pulmonary 

 artery, where having been exposed to the action of the air 

 and becoming greatly changed in its composition, it returns 

 to the heart by means of the pulmonary veins, emptying 

 itself into the left auricle, passing from here through the 

 auriculo-ventricular opening into the left ventricle, and 

 from thence into the aorta to be pumped all over the body 

 to which it is distributed by means of the arteries and 

 capillaries ; it is then collected by the veins, and eventually 

 brought back to the heart to undergo afresh its distribu- 

 tion to lungs and body. The use of the valves is to allow 

 of the transference of blood from auricle to ventricle, 

 and from the ventricles to the aorta and pulmonary artery 

 without any chance of regurgitation. 



The heart occupies a position in the middle line of the 

 chest, being suspended from the spine by its aortic vessels. 



