264 
MANUAL OF MODERN FARRIERY. 
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 
