DISSECTION OF THE BACK AND THORAX. 109 



In its interior there are four cavities — two auricles aud two Tentricles, 

 the auricle of each side being placed above the ventricle. This sub- 

 division of the interior of the heart into cavities is indicated on its 

 exterior by certain grooves. Thus, the auricido-ventricular groove runs 

 around the heart like a belt, and marks off the auricles from the 

 ventricles. Although this groove is carried quite round the heart, it 

 is not very evident in front, being concealed there by the origins of the 

 aorta and pulmonary artery. Two other grooves, one on the right side, 

 the other on the left, descend from the base of the heart, and become 

 continuous a little in front of the apex. These grooves correspond to 

 the edges of the septum which separates the cavities of the i-ight side 

 from those of the left. They are much more distinctly marked on the 

 ventricular portion of the heart, where they are termed the ventricular 

 grooves. The grooves of the heart lodge the coronary vessels, and a 

 quantity of fat which is present in all but the most emaciated 

 subjects. 



In form the heart resembles a cone compressed from side to side ; 

 and its exterior may be described as presenting two surfaces, two 

 borders, a base, and an apex. The surfaces of the heart are formed by 

 the ventricles, and when these cavities are distended both surfaces are 

 convex. The right side of the heart is formed principally by the right 

 ventricle, but partly also by the left. The right ventricular groove 

 descends on it, and crosses round the anterior border a little above the 

 apex. The left side belongs chiefly to the left ventricle, but partly in 

 front to the right. It shows the left ventricular furrow crossing 

 towards the anterior border, where it joins the furrow of the opposite 

 side. The anterior border of the heart is slightly convex, and has an 

 oblique direction downwards and backwards when the heart is in position. 

 It belongs nearly altogether to the right ventricle, the two ventricular 

 furrows, which denote the position of the septum between the two 

 cavities, becoming continuous round this border a little above the apex. 

 The posterior border is thicker and less flaccid than the anterior. It is 

 nearly straight, and is disposed almost vertically when the heart is in 

 position. It belongs entirely to the left ventricle. The base of the 

 heart lies above, and is formed by the auricles. At this point the 

 large vessels pass to and from the heart, and form the principal 

 means of its suspension. The left auricle forms the left posterior 

 part of the base, and consists of a sinus venosus into which the 

 pulmonary veins open, and an ear-shaped appendix— the auricula, the 

 latter being most posterior. The right auricle forms the right anterior 

 part of the base, and also consists of a sinus venosus and an auricula, 

 the latter lying in front. The large systemic veins discharge them- 

 selves into the sinus venosus of the right auricle. The apex of the 

 heart is blunt and firm, and belongs to the left ventricle. 



