THE RIGHT VENTRICLE 623 
Although called a fossa, it is usually a deep but narrow diverticulum in the horse. The 
interatrial septum is very thin here, and in some cases the foramen ovale fails to close entirely. 
THE RIGHT VENTRICLE 
The right ventricle (Ventriculus dexter) constitutes the right-anterior part of 
the ventricular mass. It forms almost all of the anterior border of the heart, but 
does not reach the apex, which is formed entirely by the left ventricle. It extends 
from the third rib to the fourth intercostal space on the left side, to the fifth inter- 
costal space on the right side. It issomewhat triangular in outline, and is crescentic 
in cross-section. Its base is connected largely with the right atrium, with which it 
Pulmonary veins 
‘ / 
_ A’ 
7 
~ 
Anterior _-- Ym 
1 V0 
vena cava Great coronary 
vein 
/ Left coronary 
_ artery (circum- 
vs - flex branch) 
Ria} , W = -—— Bicuspid valve 
i -~-* 
OFriun » Sy 5) _-Chorde tendinee 
_. Musculus 
papillaris 
_~ Moderator band 
Right coronary 
artery 
: ¥* x zeney _--~ Left ventricle 
Chorde tendinee “SX 1. j : : : b- 
Ey 
Moderator band-- aay 
Right coronary artery --7 
Fic, 547.—Section oF Heart oF Horse. 
Specimen hardened in situ and cut nearly at right angles to the ventricular septum. The left ventricle is contracted, 
but not ad marimum. V.a., Segment of aortic valve. 
communicates through the right atrio-ventricular orifice; but its left part projects 
higher and forms the conus arteriosus, from which the pulmonary artery arises. 
Its apex is two inches or more (ca. 5-6 em.) above the apex of the heart. On 
opening the cavity it is seen that the atrio-ventricular orifice and the cavity of the 
conus arteriosus are separated by a thick rounded ridge (Crista supraventricularis). 
The axis of the cavity, taken from this ridge to the apex, forms a spiral curve down- 
ward and to the right. The septal wall is convex and faces obliquely forward and 
to the right. 
The right atrio-ventricular orifice (Ostium atrio-ventriculare dextrum) is oval 
and is chiefly opposite to the fourth and fifth ribs and the intervening space. The 
plane of the opening is oblique, much lower in front than behind. 
The extent of the atrio-ventricular orifices, of course, varies with the phase in which the heart 
is fixed. In subjects which are preserved by intravascular injection of formalin solution the night 
side of the heart is usually in diastole, while the left side is more or less contracted. 
