CHAPTER IV. 
THE CIRCULATORY SYSTEM. 
The circulation of the blood is carried on by means of the heart, 
arteries, capillaries, and veins. 
Arteries. — The arteries are the channels which convey blood 
from the heart, distributing it over the system. If an artery be 
pressed by the finger, an impulse or shock is felt, known as the 
pulse. They aid in forcing the blood onwards by means of their 
elastic walls, which are thicker than those of veins, and do not 
collapse when empty. 
Capillaries. — Capillaries are the very minute channels which 
allow certain portions of the blood to pass through their walls into 
the surrounding tissues for their nourishment. They are the anas- 
tamotic network of vessels whereby the terminations of the arteries 
are continued to form the beginning of the initial branches of the 
veins. 
Veins. — Veins are the channels which carry blood back to the 
heart. 
The small veins unite to form main trunks which finally carry 
the venous blood to the right auricle, from which it passes into the 
right ventricle, and from thence through the artery of the lungs 
(pulmonary, the opening of which is in the ventricle) to the lungs. 
After oxygenation in the lungs it is conveyed to the left auricle, 
then to the left ventricle into the aorta, a large artery, which 
distributes the purified blood over the body, 
Lymphatic. — The lymphatic system consists of a series of tubules, 
the lymph vessels, and a number of glands. In the elephant the 
lymphatic system is so poorly developed as to be almost non-existent. 
There are few lymphatic vessels, and the lymphatic glands are few 
and small in number. The main lymphatic vessel, the thoracic duct, 
is extremely small, being not much larger than that in man. The 
lymphatic vessels are for the purpose of carrying lymph from the 
blood to the tissues. 
The heart. — The heart is a hollow muscular organ consisting of 
four chambers, situated within the chest and enclosed within a bag 
(the pericardium). 
The two upper chambers are called auricles ; the lower two^ 
ventricles. 
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