; 
THE PERICARDIUM AND HEART 743 
The area of contact of the pericardium with the chest wall is chiefly ventral, and is best seen 
with the subject on its back. In this position (after removal of the intercostal and rectus thoracis 
muscles), the area is seen to be almost triangular. The anterior border of the triangle (formed by 
the apical lobe of the right lung) is at the right fourth costal cartilage and extends across the median 
plane, ending at the third interchondral space near the costo-chondral junction. The right border 
extends from the sternal end of the fourth rib to the eighth chondro-sternal joint. The left border 
begins at the left end of the anterior border, crosses the fourth cartilage nearly an inch from its 
junction with the rib and the fifth and sixth chondro-costal junctions. 
The heart differs greatly in form and position from that of the larger animals. 
In diastole it is ovoid and the apex is blunt and rounded. Its long axis is very 
oblique. Thus the base faces chiefly toward the thoracic inlet and is opposite to 
the ventral part of the third rib. The apex is on the left side at the sixth inter- 
chondral space or seventh costal cartilage, and in close relation to the sternal part 
of the diaphragm. The sterno-costal surface corresponds largely to the floor of 
the thorax from the fifth chondro-sternal joint backward. It is crossed obliquely 
Aorta Left pul- 
monary 
artery 
Pulmonary veins 
Left brachial Posterior vena cava 
artery 
Brachiocephalic — | 
artery 
Anterior vena 
cava 
Right auricle 
Pulmonary artery Left ventricle 
Conus arteriosus 
Right ventricle 
Apex 
Fic. 612.—Herart or Dog; Lerr View. 
1, Pulmonary veins; 2, left auricle; 3, vessels and fat in coronary groove; 4, vessels and fat in left or ventral longitu- 
dinal groove; 5, vessels and fat in intermediate groove. 
by the left (or ventral) longitudinal groove, which begins behind the origin of the 
pulmonary artery, and joins the right (or dorsal) groove at the right border, thus 
forming a notch not far from the apex. The diaphragmatic surface is less ex- 
tensive and less convex. On it is the right (or dorsal) longitudinal groove, which 
begins at the coronary groove just behind the termination of the posterior vena 
cava and joins the left (or ventral) groove to the right of the apex. There is fre- 
quently an intermediate groove between these on the left ventricle, which extends 
a variable distance toward the apex. The weight of the heart of a dog of medium 
size is five to six ounces, and is about 1 per cent. of the body weight. 
_ The relative weight is subject to wide variation. It is large in hunting dogs and such as are 
trained for speed or worked. In fat dogs of sedentary habit it may be only about 0.5 per cent. : 
_ The fossa ovalis is very shallow and is bounded in front by the intervenous crest. The musculi 
pectinati form a rich network in the right auricle. The tricuspid valve has two chief cusps and 
three or four small ones. There are four musculi papillares in the right ventricle, all of which 
spring from the septal wall. They diminish in size from before backward, the anterior ones being 
long and cylindrical. Their bases are connected with the lateral wall by trabecule carnee. Usu- 
