DISSECTION OF THE BACK AND THORAX. 107 



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 foi-m tlie 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 

 obliqvie 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 a2:)€x of the 

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



Position and relations. — In order to study these, the heart should now 

 be restored to its natural position Avithin the pericardium. 



The position of the heart may be expressed with regard to the skeleton 

 as follows : — It lies beneath the bodies of the dorsal vertebrse from the 

 4th to the 10th inclusive; it responds to the four ribs behind the 2nd; 

 and it is placed above the sternum from about the 3rd chondro-sternal 

 joint to within an inch of its posterior extremity. 



In a medium-sized animal the most anterior pai-t of the heart (right 

 auricula) is distant about four or five inches from the entrance to the 

 chest ; the posterior liorder at its upper part is separated by about the 

 same interval from the tendinous centre of the diaphragm, but at its 

 lower part it is only about an inch in front of the insertion of the rim of 

 the diaphragm across the ensiform cartilage ; and during great distension 

 of the abdominal viscera, the diaphragm may be driven forwards so as 



