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 pa pillary 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 righc 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. 



