CIRCULATION. 37 



The opening between the right auricle and tlie right ventricle 

 is guarded by the right auriculo-ventricular valve ; the opening 

 between the right ventricle and ihe pulmonary artery is guarded 

 by the right semilunar valve. 



The opening between the left ventricle and the left auricle 

 is guarded by the left auriculo-ventricular valve, and the open- 

 ing to the aorta from the left ventricle is guarded by the left 

 semilunar valve. 



The pnbnonary circulation is the flow of blood which occurs 

 between the heart and lungs. 



The systemic circulation is that which occurs between the 

 heart and all the rest of the body, except the lungs. 



Arteries, veins and capillaries. — Arteries have thicker and 

 more elastic walls ; remain open after death although empty ; 

 the stream flows in jets ; the blood is lighter in color than that 

 in the veins and flows from the heart. Arteries have no valves. 

 In all these points the arteries differ from the veins. Both have 

 three coats: outer, fibrous; middle, muscular; and inner, serous. 



Capillaries are the small vessels and spaces which connect 

 minute arteries with minute veins, 



BLOOD SUPPLY OF THE BODY. 



Arteries. — The aorta is the trunk artery which receives blood 

 from the left ventricle for the systemic circuit. It is about two 

 inches long and branches into two large trunks ; viz., the anterior 

 aorta and the posterior aorta. The anterior supplies the head, 

 neck and front limbs ; and the posterior supplies in a general way 

 the rest of the body. 



Anterior aorta is smaller and shorter (one inch long), course 

 is upwards and forwards. It divides into the right and left 

 axillary arteries. These lie near the trachea, one on each side, 

 for a short distance and then bend around the anterior borders 

 of the first ribs and terminate at the inner part of each shoulder 

 in the humeral arteries, which are the continuing branches of 

 the axillary. Each humeral furnishes blood for the correspond- 

 ing front limb. 



The common carotid arteries, right and left, supply various 

 structures in the neck and head. These have their origin in a 

 single vessel, the cephalic artery which branches from the right 



