Cavity of Heart . 
883 
the heart. There is thus formed a “ bicuspid ” valve at each 
orifice. By referring to Fig. 7, PI. VI, it will be seen that 
each curtain is club-shaped in longitudinal section, being much 
narrower at its origin from the wall of the heart than at its 
free extremity. Behind each valve there is a “ sinus ” similar 
_n function to those found behind the semi-lunar valves of the 
vertebrate heart. Fig. 5, PI. VI, shows a section through the 
dorsal apertures, and Fig. 6, PI. VI, shows a section through 
the lateral apertures. 
cavity of heart. 
It will be remembered that the arteries arising from the 
anterior end of the heart are situated nearest to the dorsal 
border, while the single artery arising from the posterior end 
is nearest to the ventral border. The cavity of the heart con¬ 
forms to this arrangement, being at the anterior end nearest to 
the dorsal and at the posterior end nearest the ventral surface. 
In the anterior third of the heart, the cavity is large and slopes 
upward towards the exit of the arteries. The dorsal valvular 
apertures open into this portion of the heart. 
The anterior half of the middle third is much larger than is 
the posterior half. The anterior and posterior lateral valvular 
apertures open into this part of the heart. The narrowing of 
the cavity begins immediately behind the posterior valvular 
orifice. 
There are no valvular apertures in the posterior third; the 
cavity does not slope down to the single arterial opening, but 
ends in the funnel-shaped extension which has already been 
described. (Figs. 2 and 3, PI. V.) 
The walls of the cavity (the ventricle) are made up of muscle 
fibres covered by a thick layer of epithelium. The muscle fibres 
are arranged in layers which run in various directions, inter¬ 
lacing with each other. Two quite prominent layers may be 
noted, one longitudinal in direction and one circular; the longi¬ 
tudinal is the most external. The epithelium covering the mus¬ 
cular wall is composed of several layers of large irregular-shaped 
cells. Fig. 7, PI. VI, gives a view of the cells and the relation 
which they bear to the muscle. 
