THE HEART 235 



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 originally from two main trunks, the pul- 

 monary 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 extremely irregular growth. It takes 

 its rise from the left ventricle, 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 dimen- 

 sions and calibre than the aorta. It has its origin in 

 the posterior 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 sympatheti- 



