618 BLOOD-VASCULAR SYSTEM OF THE HORSE 
dorsal and middle thirds of the dorso-ventral diameter of the thorax. It is opposite 
to the lateral wall of the thorax, from the second intercostal space or third rib to the 
sixth rib or intercostal space.!. The apex (Apex cordis) lies centrally above the last 
segment of the sternum; it is half an inch or less (ca. 1 em.) above the sternum and 
about an inch (ca. 2-8 em.) from the sternal part of the diaphragm. The anterior 
border (Margo cranialis) is strongly convex and curves ventrally and backward; 
the greater part is parallel with the sternum. The posterior border (Margo cau- 
dalis) is much shorter, is nearly vertical, and is opposite to the sixth rib and inter- 
costal space. The surfaces, right and left (Facies dextra, sinistra), are convex and 
Lig. arteriosum Pulmonary veins 
Left auricle 
Anterior 
vena cava 
Coronary 
groove 
Right 
auricle Posterior 
border 
Coronary 
groove 
Left longitudinal groove 
Fic. 542.—Hearr or Horse; Lert View. HARDENED in situ 
The dotted line indicates the line of reflection of the serous pericardium. The epicardium and subepicardial fat have 
not been removed, 
are marked by grooves which indicate the division of the heart into four chambers, 
the two atria above and two ventricles below. The left surface (covered by the 
pericardium) is related to the lower third of the chest-wall from the third to the 
sixth rib. On the right side the cardiac notch of the lung is smaller, so that the 
area of relation to the chest-wall extends from the third to the fourth intercostal 
space.” 
‘The size and form of the heart vary according to the degree of its contraction and relaxa- 
tion (systole and diastole). In subjects which have been bled and preserved by intravascular 
injection of formalin solution the right side is usually fixed in diastole while the left is more or less 
strongly contracted. The base may extend back to the seventh rib. 
? The arrangement in this regard has been stated at greater length in the descriptions of the 
lungs and the pericardium. 
| 
