248 DISEASES OF THE HORSE. 
ture, the auriculo-ventricular orifice, which is furnished with a 
remarkable mechanism of valves, allowing the transmission of blood 
from the auricles into the ventricles, but preventing a reverse course. 
The ventricles are thick-walled cavities, forming the more massive 
portion of the heart toward the apex. They are separated by a par- 
tition, and are connected with the great arteries—the pulmonary 
artery and the aorta—by which they send blood to all parts of the 
body. At the mouth of the aorta and at the mouth of the pulmonary 
artery is an arrangement of valves in each case which prevents the 
reflux of blood into the ventricles. The auriculo-ventricular valve 
in the left side is composed of two flaps, hence it is called the 
bicuspid valve; in the right side this valve has three flaps and is 
called the tricuspid valve. The flaps which form these valves are 
connected with a tendinous ring between the auricles and ventricles; 
and each flap of the auriculo-ventricular valves is supplied with 
tendinous cords, which are attached to the free margin and under- 
surface, so as to keep the valves tense when closed—a condition which 
is produced by the shortening of muscular pillars with which the 
cords are connected. The arterial openings, both on the right and 
on the left side, are provided with three-flapped semilunar-shaped 
valves, to prevent the regurgitation of blood when the ventricles 
contract. The veins emptying into the auricles are not capable of 
closure, but the posterior vena cava has an imperfect valve at its 
aperture. 
The inner surface of the heart is lined by a serous membrane, the 
endocardium, which is smooth and firmly adherent to the muscular 
structure of the heart. This membrane is continuous with the lining 
membrane of the blood vessels, and it enters into the formation of 
the valves. 
The circulation through the heart is as follows: The venous blood 
is carried into the right auricle by the anterior and posterior vene 
cave. It then passes through the right auriculo-ventricular opening 
into the right ventricle, thence through the pulmonary artery to the 
lungs. It returns by the pulmonary veins to the left auricle, then is 
forced through the auriculo-ventricular opening into the left ven- 
tricle, which propels it through the aorta and its branches into the 
system, the veins returning it again to the heart. The circulation, 
therefore, is double, the pulmonary, or lesser, being performed by the 
- right side, and the systemic, or greater, by the left side. 
As the blood is forced through the heart by forcible contractions of 
its muscular walls, it has the action of a force pump, and gives the 
impulse at each beat, which we call the pulse—the dilatation of the 
arteries throughout the system. The contraction of the auricles is 
quickly followed by that of the ventricles, and then a slight pause 
occurs; this takes place in regular rhythmical order during health. 
