126 THE ANATOMY OP THE HORSE. 



Lower. 2. Above and in front of the opening of the posterior cava is a 

 depression of the septum that looks like another venous orifice. This is 

 the fossa ovalis, and it marks the former position of the foramen ovale — 

 an aperture which, in the fcBtus, established a communication between 

 the right and left auricles. The raised border which surrounds the 

 fossa is termed the annulus ovalis. In the foetus of many animals, 

 but not of the horse, a valve, termed the Eustacliian valve, directs the 

 blood from the posterior cava through the foramen ovale. After birth 

 the foramen ovale in nearly every case becomes completely closed, but 

 sometimes an oblique sKt remains, which, however, does not necessarily 

 permit any blood to pass through the septum. 



The blood which passes through the right auricle is venous in char- 

 acter. It has been circulating among the tissues, and it is poured into 

 the cavity at the venous orifices already enumerated. When the 

 auricle contracts, the blood is passed into the ventricle of the same side, 

 by a large aperture of communication between the two cavities — the 

 right auriculo-ventricular opening. 



Directions. — The fore and middle fingers of the left hand should be 

 introduced through the auriculo-ventricular opening, so as to grasp, 

 between the fingers and thumb, the wall of the right ventricle close to 

 the angle of junction between the right ventricular and the auriculo-ventri- 

 cular groove. The scalpel should then with the right hand be passed 

 through the wall of the ventricle at that point, and carried downwards 

 in front of the right ventricular furrow ; and following that furrow 

 round the anterior border of the heart, the incision should be continued 

 up in front of the left ventricular furrow, as far as the root of the 

 pulmonary artery. This will enable nearly the entire wall of the right 

 ventricle to be raised as a triangular flap, and will give a good view of 

 the cavity when looked into from below. 



The Cavity of the Eight Ventbiclb. This cavity is widest above, 

 and tapers to its lowest point ; and its shape is such that its transverse 

 section gives a crescentic outline, the wall of the ventricle being concave 

 towards the cavity, while the septum is convex in the same direction. 

 The inner surface of its wall is rendered irregular by muscular bands 

 and prominences — the columnce carnece, of which there are several 

 varieties : 1. Some of them have the form of bars or ridges sculptured 

 on the wall of the heart, to which they give a sponge-like appearance. 

 2. Others, the trabeculas cameos, are veritable bands or strings between 

 which and the wall of the ventricle the handle of a scalpel may be 

 passed. Of this variety two or three very tendinous strings, sometimes 

 more or less reticulate, stretch between the wall and the middle of the 

 septum ; and since they are believed to prevent over-distension of the 

 ventricle, they have been named moderator bands. Other strings occur 

 in the angle of junction of wall and septum, and still others stretch 



