THE VASCULAR SYSTEM. 
The vascular system is composed of the central forcing 
muscular organ, the heart; a system of vessels, the arteries, 
carrying the blood from the ventricles to the lungs and all 
parts of the body; a system of vessels, the veins, returning 
the blood to the auricles; and numerous microscopic vessels, 
the capillaries, connecting the termination of the arteries 
with the origin of the veins. 
THE HEART. 
The location of the heart in the cat may be seen by re- 
moving the ventral thoracic wall, but for the study of the 
parts, the heart of an ox or a sheep will be found more 
satisfactory. The heart of the cat lies between the lungs 
(Fig. 82) in the thoracic cavity a little to the left of the 
median line. The caudal end is the apex, and the cranial 
end is the base. The entire organ is invested by a tough 
membrane, the pericardium, which when cut permits the 
pericardial fluid to run out. All the blood-vessels originate 
from the dorsocranial aspect of the heart. 
It is composed of a right and left half, each of which 
consists of an auricle and a ventricle. The separation be- 
tween the two halves is apparent on the ventral surface. 
The auricles receive the blood from the veins and pass it 
to the ventricles, which disperse it through the arteries. 
There is no aperture between the auricles or the ventricles. 
The aperture between the right auricle and the right ven- 
tricle is guarded by the tricuspid valve which prevents the 
blood from returning into the auricle when the systole or 
contraction pushes the blood into the pulmonary artery. 
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