46 COMPARATIVE ANATOMY 



membrane, which is reflected over the heart, and so excludes 

 it from the cavity which it lines. 



The heart itself consists of (i) two auricles, forming the 

 wider anterior portion : they are readily distinguished by 

 their thinner walls and darker colour, due to the blood being 

 seen through their walls ; (2) a single ventricle, situated 

 posteriorly, with thick muscular walls, which are of a paler 

 colour than those of the auricles; (3) the sinus venosus, a 

 thin-walled sac, lying on the dorsal or hinder region of the 

 heart, and formed by the union of three great veins ; (4) the 

 truncus arteriosus, a stout cylindrical vessel which arises from 

 the ventral surface of the right-hand side of the base of the 

 ventricle. Examining the structure of the heart more closely 

 we find that the sinus venosus is a thin-walled sac of triangular 

 shape, the apex of the triangle pointing backward. At its 

 apex it receives a large blood-vessel, the inferior vena cava, 

 and at each of its remaining angles, right and left, it receives a 

 large vessel, the right and left superior vense cavse. The sinus 

 venosus communicates by a transverse opening, guarded by 

 two valves, with the right auricle. The valves at this opening 

 are so disposed that they admit blood from the sinus venosus 

 into the auricle, but prevent its passage in a reverse direction. 



The two auricles together form a large thin-walled sac, of 

 rather irregular but generally hemispherical shape, which 

 opens below by an elongated slit into the ventricle. The sac 

 is divided into two unequal parts by the auricular septum 

 passing from the anterior wall to the auriculo-ventricular 

 aperture, where it ends in a free concave border. Of the two 

 auricles the left is decidedly the smaller, and in exceptional 

 cases it is much smaller than the right. It receives at its 

 upper dorsal surface, close to the auricular septum, a vein the 

 pulmonary vein which brings blood back from the lungs. 



The ventricle is much thicker than the auricles, through the 

 presence of abundant muscular tissue in its walls. Its cavity 

 is not smooth, but is rendered spongy in appearance, by the 

 presence of numerous muscular ridges which project into it. 

 The cavity does not extend far into the apex of the ventricle, 

 is rather narrow dorso-ventrally, but elongated from right to 

 left. The auriculo-ventricular aperture, by which the ventricle 

 communicates with the auricles, is rather a wide opening, 

 divided into two by the free edge of the auricular septum, and 



