THE HEART. 461 



nearer to the left border of the heart, and the latter to the right. It follows, there- 

 fore, that the right ventricle forms the greater portion of the anterior surface of 

 the heart, and the left ventricle more of its posterior surface. 



Each of these cavities should now be separately examined. 



The Right Auricle is a little larger than the left, its walls somewhat thinner, 

 measuring about one line ; and its cavity is capable of containing about two ounces. 

 It consists of two parts: a principal cavity, the sinus venosus, or atrium, situated 

 posteriorly, and an anterior, smaller portion, the appendix auriculae. 



Bristle passed thro 

 right auricula-ventricular opening. 



FIG. 276. The right auricle and ventricle laid open, the anterior walls of both being removed. 



The sinus is the large quadrangular cavity, placed between the two venge 

 cavae ; its walls are extremely thin ; it is connected below with the right ventricle, 

 and internally with the left auricle, being free in the rest of its extent. 



The appendix auriculae, so called from its fancied resemblance to a dog's ear, 

 is a small conical muscular pouch, the margins of which present a dentated edge. 

 It projects from the sinus forward and to the left side, overlapping the root of the 

 aorta. 



To examine the interior of the right auricle, an incision should be made along its right 

 border, from the entrance of the superior vena cava to that of the inferior. A second cut is to 

 be made from the centre of this first incision to the tip of the auricular appendix, and the flaps 

 raised. 



The internal surface of the right auricle is smooth, except in the appen- 

 dix and adjacent part of the anterior wall of the sinus venosus, where the mus- 

 cular wall is thrown into parallel ridges resembling the teeth of a comb and 

 hence named the muscuU pectinati. These end behind on a vertical smooth 

 ridge, the crista terminalis of His, the position of which is indicated on the sur- 

 face of the distended auricle by a furrow, the sulcus terminalis (His) ; this rep- 

 resents the line of fusion of the sinus venosus of the embryo with the primitive 

 auricle proper. 



