90 Veterinary Elements. 



openings, one from the right auricle, the other into the 

 pulmonary artery. The course of the blood through the 

 heart is as follows : 



Starting in the right auricle it flows to the right ventri- 

 cle, thence to the lungs by the pulmonary artery, where it 

 will be remembered oxygen (O) is exchanged for carbonic 

 acid (CO), it is then returned, purified, by the pulmon- 

 ary vein to the heart, this time being poured into the 

 left auricle, from which it passes to the left ventricle 

 and thence by means of the aorta to all parts of the body. 

 Prom the various parts of the body the blood is brought 

 back by small veins which unite to form larger ones 

 until it is again emptied by the vena cava into that part 

 of the heart known as the right auricle, the point from 

 which the start was made, thus the blood has made the 

 entire circuit. Harvey was the first man to thoroughly 

 understand and describe the circulation of the blood. 



Arteries and veins form the channels for the circula- 

 tion of blood, the arteries taking the blood from the 

 heart, the veins fetching it towards the heart. These 

 tubes or channels are made up of three coats, differences 

 in which aid us in determining whether we are looking 

 at an artery or vein. The inner coat of an artery is com- 

 posed of endothelium and some elastic tissue, the middle 

 coat is made up of muscular and elastic fibres; the outer 

 coat, very strong, is formed of connective tissue and 

 elastic fibres, it is far less easily torn than the other 

 coats and is more resistant to pressure. Arteries as a 

 rule are deeply seated in muscles to avoid injury, com- 

 municate freely with one another (anastomosis) and are 

 always accompanied by a vein. The inner and middle 



