126 THE ANATOMY OP THE HORSE. 



right of its apex ; the horizontal branch terminates in the same way 

 near the posterior border of the heart. 



The corresponding branches of the right and left arteries, thus, 

 approach each other at their terminations, but they do not anastomose ; 

 nor is there any anastomosis between the arteries through their colla- 

 teral branches. Still more, there is no anastomosis between the adjacent 

 collateral branches of the same coronary artery. 



The Coronary FeiW.— These arise from the capillaries of the coronary 

 arteries. The principal vessel of the right side ascends in the right 

 ventricular groove, and at the auriculo-ventrioular furrow it joins the 

 main vein of the left side. The latter ascends at first in the left ventri- 

 cular furrow, at the top of which it enters the auriculo-ventricular 

 groove. Along this it is reflected backwards ; and, turning round the 

 posterior border of the heart, it joins the right vein. The dilated vessel 

 resulting from this union is termed the coronary venous sinus, and it 

 opens into the right auricle, beneath the mouth of the posterior vena 

 cava. 



For the most part, the veins arising in the wall of the right auricle 

 do not join the large coronary veins, but open into the cavity indepen- 

 dently, by minute mouths — the foramina Thebesii. 



The Nerves of the Heart have already been described (pages 112 and 

 119). In the heart of a lean subject long filaments are visible without 

 dissection, descending beneath the serous covering. 



The Interior of the Heart. — The cavities of the heart should be 

 studied in the order in which the blood passes through them, and 

 therefore the right auricle falls to be examined first. The termina- 

 tions of the anterior and posterior cavte and vena azygos (provided 

 that has an independent opening) should be identified ; and then an 

 incision should be made along the wall of the sinus venosus, from 

 the opening of the anterior to that of the posterior cava. Another 

 incision should be carried from this one to the point of the auricula ; 

 and, the clots of blood having been cleared out, the cavity will be ready 

 for examination. 



The Cavity of the Right Auricle. The interior of this and the 

 other cavities of the heart is smooth and glistening in virtue of an 

 endothelial membrane termed the endocardium, which is here continuous 

 with the endothelial lining of the great veins. It will be observed that 

 the muscular wall of the auricle is thrown into parallel ridges, which 

 from their resemblance to the teeth of a comb are termed musculi 

 pectinati. The venous orifices by which the blood is poured into the 

 cavity are all found in the sinus venosus, and are as follows: — 1. The 

 anterior vena' cava empties itself into the anterior part of the roof of 

 the sinus. 2. The posterior vena cava discharges itself at the lower 

 and back part of the outer wall of the sinus. 3. The coronary venous 



