THE THORAX. 261 
Open the right ventricle by a longitudinal incision of the ventral 
wall, extending the incision forward into the pulmonary artery, 
also both atria by transverse incisions. By washing out the cavi- 
ties, the internal features of the wall, including the arrangement of 
the valvular structures, may be examined as follows: 
In the right ventricle: 
(a) The trabeculae carneae; muscular ridges of the internal 
surface of the wall. 
(b) The tricuspid valve (valvula tricuspidalis). The thin 
membranous flaps composing the valve enclose the atrio- 
ventricular aperture, and project into the cavity of the 
ventricle. Their margins, which are otherwise free, are con- 
nected by slender fibrous cords, the chordae tendineae, 
with the papillary muscles (mm. papillares), the latter 
being thick muscular projections, of somewhat conical shape, 
arising from the opposite walls. 
In the rabbit the valve is composed of only two flaps, of which the 
ventral one is very free, and has large papillary muscles, while the dorsal 
one is closely attached to the wall, with the papillary muscle reduced or 
absent. For this reason the term right atrioventricular valve is more 
appropriate than “‘tricuspid’’. 
(c) The semilunar valves (valvulae semilunares) of the pul- 
monary artery are three extremely thin folds guarding the 
entrance to the vessel from the right ventricle. Two of the 
valves are usually found intact, the third being destroyed on 
opening the vessel. 
In the atria: 
(a) The respective positions of the pulmonary and_ systemic 
veins at their points of entrance. 
(b) The complete separation of the two chambers. In the par- 
tition separating them may be seen a thin fibrous portion 
denoting the position of the embryonic foramen ovale. 
Open the left ventricle by a ventral longitudinal incision, cutting 
well through the tip of the ventricle and extending the incision 
across the pulmonary artery and into the aorta. On account 
of the great thickness of the wall the internal structure is not so 
easily examined as in the right ventricle. 
