264 MANUAL OF MODERN FARPJERY. 



cells, there to undergo the important change of being 

 subjected to the action of the atmospheric air which the 

 lungs have inhaled, and be purified by the oxygen contained 

 in the air, and from which substance it owes its beautiful 

 red colour. It is now carried to the left auricle, and from 

 thence it descends to the left ventricle, and by the powerful 

 closing of the ventricle is propelled into the arteries. These 

 vessels, in all their numerous ramifications, emanate origi- 

 nally from two main trunks, the pulmonary artery and the 

 aorta ; the branches of the former penetrate the lungs, and 

 the latter are spread over every part of the body. 



The aorta, with its numerous branches, when taking them 

 as a whole, may be compared to a short but straggling and 

 very branching shrub or dwarf tree, of luxuriant but ex- 

 tremely irregular growth. It takes its rise from the left ven- 

 tricle, and the blood by the force communicated to it by the 

 sudden contraction of the ventricle, and aided by the elastic 

 power of the arteries, keeps them open and free from 

 obstruction, and likewise, by the pressure of the muscular 

 and elastic coats, endeavouring to return to their former 

 dimensions, flows in a continuous stream through every 

 portion of the frame. 



The pulmonary artery is a vessel of larger dimensions 

 and calibre than the aorta. It has its origin in the pos- 

 terior upper part of the right ventricle of the heart, and 

 winding upwards to the root of the left lung, there divides 

 into what are termed the right and left pulmonary arteries. 

 These divisions immediately enter the substance of their 

 correspondent lungs, and therein ramify in all directions, 

 like the minutest threads, the branches regulating their 

 course and division by the ramification of the bronchial 

 tubes. 



The heart is liable to disease, as it is sympathetically 



