332 ANATOMY OF THE RABBIT 



In the most primitive vertebrate hearts, the ventricular wall is 

 composed almost entirely of a spongy mass of muscular trabeculae 

 with a thin layer of compact muscle on the outer surface. There is a 

 progressive change in the vertebrate series to the mamm.alian condition, 

 where there is a thick, compact muscular wall with relatively few 

 internal trabeculae. 



(b) The tricuspid valve (valvula tricuspidalls). 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 

 connected 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 re- 

 duced 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. Each fold 

 forms a pocket opening towards the artery and the cavity 

 of the pocket is a pulmonary sinus. 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 

 partition separating them may be seen a thin fibrous por- 

 tion, the fossa ovalis, denoting the position of the embry- 

 onic 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 greater thickness of the wall the internal structure is not so 

 easily examined as in the right ventricle. The interventricular 



