226 DISEASES OF THE HORSE. 



body. At the mouth of the aorta and at the month of the puhnonary 

 artery is an arrangement of valves in each case which prevents the 

 reflux of blood into the ventricles. The auriculo-ventricular valves 

 in the left side are composed of two flaps, hence it is called the 

 hicuspid 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 venae 

 cavae. 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. 



The action of the heart is governed and maintained by the pneumo- 

 gastric nerve (tenth pair of cranial nerves) ; it is the inhibitory nerve 

 of the heart, and regulates, slows, and governs its action. When the 

 nerve is cut, the heart-beats increase rapidl}^, and in fact the organ 

 works without control. "Wlien the nerve is unduly irritated the hold- 

 back, or inhibitory force, is increased, and the heart slows up in the 

 same measure. The left cavities of the heart, the pulmonary veins, 



