SHEEP HUSBANDRY IN THE SOUTH. 235 



discharged into the right auricle, and thence into the right ventricle. By 

 the contraction of the latter, its contents are forced through the pulmona- 

 ry artery into the lungs. The blood having been purified in the lungs, is 

 returned to the left auricle ; thence into the left ventricle ; and it is then 

 forced into the aorta, or large artery which supplies, by its different 

 branches, all parts of the system with blood. Each compartment of the 

 heart is furnished with appropriate valves to cause the blood to be forced 

 forward in its regular course, by the muscular contractions of this viscus. 

 These contractions are the result of an inherent and independent power. 



The contractions of the heart force the blood into and along the arteries. 

 When this force begins to be spent as the distance from the heart in- 

 creases, it receives aid from the action of the muscular coat of the arteries 

 themselves, which forces along the blood to their utmost extremities. 



The arteries continue to branch off into more and more minute divisions 

 as they recede from the heart, until the tubes are much less in diameter 

 than the finest hair. These, capillaries as they are called, open by exceed- 

 ingly minute mouths in every part of the frame, for the deposition of those 

 secretions from the blood which maintain the vitality and healthy action 

 of the parts, supply the animal waste, &c. 



The capillaries, commencing their return toward the heart, constantly 

 reunite, forming larger tubes which are called veins, which bring back 

 such portions of the blood carried out by the arteries, as has not been ex- 

 pended in nourishing the system. The blood now deprived of its oxygen, 

 and loaded with carbon, is unfit for farther circulation until re-purified in 

 the lungs. It is of a darker color than the arterial blood. It is no longer 

 urged on by the contractile power of the tubes through which it flows, but 

 by the partial vacuum formed in the right auricle (as at each contraction 

 it forces its contents into the right ventricle,) and by atmospheric 

 pressure. 



THE LUNGS. The lungs are bodies composed of separate minute air- 

 cells, communicating with the bronchial tubes, or subdivisions of the wind- 

 pipe. They also contain many arteries, and veins. On the delicate mem- 

 braneous walls of the air-cells the venous blood is carried by innumerable 

 tubes so thin as to permit their contents to be acted upon by the atmos- 

 pheric air which fills the cells at every inspiration. Here the blood gives 

 off its carbon, and receives oxygen from the air, and thus is prepared for 

 its return to the heart, and to be again sent through the system. 



The right lung is somewhat larger than the left, and both fill their re- 

 spective cavities when inflated. They are entirely free from any attach- 

 ment to the pleura the membrane which lines the ribs when in their 

 natural state. "When the animal has been bled to death, the lungs are of 

 a light color ; but if the animal has died with all its blood in it, their color 

 resembles that of the liver. This can, however, be readily distinguished 

 from hepatization the result of certain diseases as will be hereafter 

 shown. 



THE WINDPIPE, LARYNX, PHARYNX, &c. The bronchial tubes constant- 

 ly uniting as they approach the anterior portion or root of each lung, final- 

 ly form a single large tube, as they make their exit from each lobe, and 

 these, uniting into one, form the windpipe. This is a well known cartila- 

 ginous tube which passes out of the chest between the first two ribs, and 

 ascends on the front part of the neck. It unites with the larynx, which 

 continues the air passage from the lungs to the mouth. The (Esophagus 

 leaves the chest close beside the windpipe, and ascends the neck on the 



