130 PRINCIPLES OF VETERINARY SCIENCE 



The common brachiocephalic artery is the name of the thoracic 

 division of the aorta. In the horse it is directed forward and 

 upward to supply the fore limbs, neck, and head. 



The posterior aorta or abdominal division of the aorta, arches 

 backward and pierces the diaphragm. It supplies branches to 

 the walls and viscera of the abdominal cavity, the body muscles, 

 and the pelvic organs and limbs. 



Capillaries are the minute, microscopic continuations of the 

 arteries that connect the latter with the veins. In many cases 

 they are not much larger in diameter than a red blood-corpuscle. 

 Their walls often consist of but one layer of cells through which 

 the interchange of gases between the blood and the tissues takes 

 place. The sectional area of the capillaries is about seven hun- 

 dred times that of the aorta. 



Veins are satellites of the arteries and conduct the blood back 

 to the heart from the tissues. They have thinner walls than the 

 arteries, are less elastic, and collapse when not filled with blood. 

 In many veins the internal coat is folded up to form valves, the 

 free edges of which are directed toward the heart to prevent the 

 blood from flowing backward. The venous system has a capa- 

 city two or three times greater than the arterial. Beginning at 

 the capillaries the veins unite to form larger and larger tubes as 

 they approach the heart until the pulmonary veins are formed 

 on the left and the vense cavse on the right. 



The pulmonary veins carry the blood which has passed through 

 the lungs and has become oxygenated to the left auricle. 



The anterior vena cava carries the blood returned from the 

 head, neck, and thoracic limbs to the right auricle. 



The posterior vena cava is the largest vein in the body and 

 conveys nearly all the blood from the liver, spleen, intestines, 

 other abdominal and pelvic organs, and the pelvic limbs to the 

 right auricle. 



Lymphatics consist of a network of connecting lymph channels 

 and groups of lymph-glands. 



The lymph channels are not continuous vessels, like the arter- 

 ies, so should not be thought of as comprising a complete circu- 

 latory system for the lymph as do the arteries and veins for the 

 blood. The movement of the fluid which they carry is wholly 

 centripetal and therefore comparable in direction with the flow 

 of blood in the veins. The lymph is formed in the tissues and 



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