84 EQUINE ANATOMY. 



of four to eight pulmonary veins, which have no valves. The 

 obliterated foramen ovale is also seen on the inter-aricular septum. 



LEFT VENTRICLE. 



The walls are on an average li to if inches in thickness ; at the 

 apex they are very thin and form the point of the heart. 



The auriculo-ventricida7' opening, similar to that on the right 

 side, is closed by the initral valve, composed of two segments at- 

 tached to chordae tendinae. 



The aortic opening is similar to the one on the right, and is 

 closed by the semilunar valves. 



Structiii'e. 



Four fibrous rings around the openings. 



At the junction of the aortic and auriculo-ventricular rings is a 

 cartilaginous body. 



The muscular tissue is striped and involuntary, but unlike all 

 voluntary tissue, it inosculates and branches freely, has no sarco- 

 lemma, and its striae are not well marked. 



The auricles have two sets of fibres, one common to both, the 

 other of fasciculi arranged in loops, rings, or as sphincters. 



The ventricles have also two sets, one intrinsic running from the 

 rings obliquely around the axis of the ventricle, the other common 

 to both, running downward and from left to right, terminating in- 

 ternally in the columna carnece. 



The endocardium is a serous membrane lining the cavities of 

 the heart, having a basement membrane covered with flattened 

 endothelium, polygonal in form, each with a nucleus. Its redu- 

 plications form the valves. 



Blood supply. 



Coronary arteries, from trunk of the aorta, each following the 

 horizontal and vertical furrows. The venous blood is returned to 

 the right auricle by the coronary vein. 



Nerves. 



From pneumogastric and sympathetic. Probably also intra- 

 cardiac sympathetic ganglia. 



ARTERIES. 



The arteries are the tubes that carry arterial blood from the 

 heart over the body. 



The pubnonary artery carries venous blood to the lungs, form- 



