DISEASES OF THE HEART, BLOOD VESSELS, AND 
LYMPHATICS. 
By M. R. Trumsowesg, V. 8. 
{Revised by Leonard Pearson, B. S., V. M. D.] 
ANATOMY AND PHYSIOLOGY OF THE HEART AND BLOOD 
VESSELS. 
(Pls. XX and XXI.) 
The heart is a hollow, muscular organ, situated a little to the left 
of the center of the chest. Its impulse is felt on the left side on 
account of its location and from the rotary movement of the organ 
in action. It is cone-shaped, with the base upward; the apex points 
downward, backward, and to the left side. It extends from about 
the third to the sixth ribs, inclusive. The average weight is about 
7 to8 pounds. In horses used for speed the heart is relatively larger, 
according to the weight of the animal, than in horses used for slow 
work. It'is suspended from the spine by the large blood vessels and 
held in position below by the attachment of the pericardium to the 
sternum. It is inclosed in a sac, the pericardium, which is com- 
posed of ‘a dense fibrous membrane lined by a delicate serous mem- 
brane, which is reflected over the heart; the inner layer is firmly 
adherent to the heart, the outer to the fibrous sac, and there is an 
“intervening space, known as the pericardial space, in which a small 
amount of serum—a thin translucent liquid—is present constantly. 
The heart is divided by a shallow fissure into a right and left side; 
each of these is again subdivided by a transverse partition into two 
compartments which communicate. Thus there are four cardiac 
cavities—the superior, or upper, ones called the auricles; the inferior, 
or lower, ones the ventricles. These divisions are marked on the 
outside by grooves, which contain the cardiac blood vessels, and are 
generally filled with fat. 
The right side of the heart may be called the venous side, the left 
the arterial side, named from the kind of blood which passes through 
them.‘ The auricles are thin-walled cavities placed at the base, and 
are connected with the great veins—the ven cave and pulmonary 
veins—through which they receive blood from all parts of the body. 
The auricles communicate with the ventricles each by a large aper- 
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