448 HEALTH AND DISEASE 



example, the aorta in a rabl>it is tied al)ove its division into the two 

 iliacs, thus depriving the lower limbs of their whole supply of blood, the 

 legs soon begin to drag, and become permanently paralysed, though even 

 then, if the limbs are kept warm and preserved from injury, by placing 

 the animal on cotton-wool, a secondary circulation through collateral vessels 

 above and below the point of ligature may in course of time become estab- 

 lished, and the power and action of the limb be restored. 



The pulmonary artery is a great trunk which arises from the upper and 

 left side of the right ventricle. Its orifice is guarded by three semilunar 

 valves (fig. 191, d), which are forced open at each contraction of the heart, 

 but close during its relaxation, and then completely prevent the return of 

 the blood into the right ventricle. It conducts the blood to the lungs, 

 and after a short course divides beneath the trachea into a right and left 

 branch, which accompany the respective bronchi to the lungs, where they 

 break up into many branches, and terminate in the capillary net-work that 

 surrounds the alveoli or air-cells. The j^ulmonary artery conveys impure 

 or venous blood to be aerated at the lungs from whence it is returned, 

 charged with oxygen and freed from carbon dioxide, by the pulmonary 

 veins to the left auricle. Just before the pulmonary artery divides, an 

 oblique cord (fig. 187, da) runs from it to the posterior aorta. This is 

 an obliterated blood-vessel named the " ductus arteriosus Botalli ", which 

 before birth transmitted the impure venous blood mainly returning from 

 the head and fore extremities to the trunk and hind-limbs. Immediately 

 after birth, and as a result of the change in the circulation and the entrance 

 of air into the lungs, this vessel ceases to convey blood and undergoes 

 atrophy, becoming an impervious Ijand. 



DISTRIBUTION OF THE SYSTEMIC ARTERIES 



Excepting as regards the lungs, all the arteries distributed over the 

 system have their origin in the aorta (fig. 193). This vessel, the largest 

 in the body, arises from the base of the heart, where it is continuous with 

 the left ventricle, from which it is separated by the semilunar valves. It 

 receives the blood discharged from that cavity at each contraction. 



The Aorta is a very capacious but short vessel, measuring only from 

 2 to 3 inches in length. After leaving the ventricle it soon divides into two 

 unequal parts — the anterior and the posterior aortee; but before doing so 

 it gives off two vessels, whose branches enter the substance of the heart 

 for the nourishment of its tissue. These are the right and left coronary 

 arteries:. 



Anterior Aorta. — This vessel, the smaller of the divisions of the 



