THORAX AND ABDOMEN OF THE HORSE 49 



or entirely absent in the pulmonary valves. Behind each semilunar 

 valve the wall of the artery is slightly dilated to form a sinus. 



The aortic semilunar valves (valvulse semilunares aorta?) are 

 stronger than those at the entrance to the pulmonary artery in accord- 

 ance with the greater pressure of blood that they have to withstand. 

 The three valves are arranged so that one of them is cranial in position ; 

 the others being right caudal and left caudal. From two of the sinuses 

 behind the valves, namely, cranial and left caudal, openings into the 

 coronary arteries will be readily discovered. 



The 'pulmonary semilunar valves (valvulse semilunares arterise 

 pulmonalis) are caudal, right cranial, and left cranial in disposition. 

 The sinuses behind them have no openings leading into arteries. 



The structure of the heart. — The greater part of the bulk of 

 the heart is formed by muscular tissue constituting the myocardium. 

 This is much more abundant in the walls of the ventricles than in the 

 atria, and particularly abundant in the wall of the left ventricle. 



The muscular bundles of the atria may be described as disposed in 

 two strata. The bundles of the more superficial stratum are not con- 

 fined to one atrium but pass from the wall of one cavity to the wall of 

 the other, and are mostly arranged so that they begin and end in 

 connection with the fibrous rings surrounding the atrio-ventricular 

 orifices. Some of them enter into the formation of the septum between 

 the two atria. The bundles of the deep stratum are confined to one or 

 other of the atria. Many of them begin at an atrio-ventricular 

 fibrous ring, pursue a looped course over the atrium, and end at the 

 ring from which they sprang. Other bundles surround the orifices of 

 the various veins that enter the atria, and are disposed in a circular or 

 spiral fashion. There is no difficulty in determining that these bundles 

 are continued for some distance on to the terminations of the venae 

 cavae, and also, for a shorter distance, on to the pulmonary veins where 

 these join the left atrium. Circular fibres also surround the fossa 

 ovalis. 



The arrangement of the muscle of the ventricles is more compli- 

 cated. The surface bundles may be readily traced in a spiral sweep 

 from the base of the ventricles to the apex of the heart. At the apex 

 they converge to a vortex cordis in curves that, when the heart is 

 viewed from the apex, run clock-wise. From the vortex the bundles 

 run upwards, deep in the wall of the heart, and end in a papillary 

 muscle in the ventricle opposite to that from which they originally 

 started. Underneath the superficial bundles are others that become 

 less and less spiral the deeper they lie in the wall of the ventricle, until 



their course is at right angles to the long axis of the cavity. Casual 

 4 



