We may now, with advantage, take a short survey of the chief vessels that have their origin in the heart, and which are engaged in the distribution of the blood through the system. The large arteries rarely join each other as do the veins, and the blood which traverses them always pursues the same direction. The capillaries, on the other hand, freely anastomose or unite together, and the blood they contain sometimes runs in one, sometimes in the opposite direction, through them; and as many arteries open into the same capillary net-work, this arrangement effectively prevents the serious consequences that would result in the case of obstruction or division of an artery, unless, indeed, the artery injured is a very large one, and is the parent trunk of the several arteries which open into the same capillary plexus. If, for example, the aorta in a rabbit is tied above 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 established, 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 pulmonary 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 band.