62 
JANE [. ROBERTSON. 
P. F. and P.f.)- There are no auricular valves of any kind 
except the auriculo-ventricular plug. 
Auricular Canal. — The auricular canal cannot be recog- 
nised as a special division of the heart, and it is represented 
apparently only by a narrow flattened band of musculature 
ou the rim of the auriculo-ventricular opening converging dor- 
sally on to the auriculo-ventricular plug. On dissection, this 
musculature of the auricular canal is found to be continuous 
anteriorly with the proximal part of the dorsal wall ot‘ the 
bulbus cordis, and laterally with the auricular musculature 
on the one hand and the ventricular on the other, round the 
rim of the auriculo-ventricular opening ; posteriorly it becomes 
continuous with the musculature of the interventricular septum 
at the site of attachment of the auriculo-ventricular plug to 
the rim of the auriculo-ventricular opening. 
Ventricles. — The ventricular portion of the heart is 
a thick-walled muscular structure more or less enclosed 
between the two auricles and from which the bulbus cordis 
arises anteriorly (PI. 5, figs. 1 and 2). The ventral surface 
of the ventricle presents no remarkable features. Near the 
apex there is the little fibrous band (PI. 5, fig. 7) that binds the 
heart to the pericardium (this has been dissected away in PL 
5, figs. 1 and 2). The continuity of the dorsal wall of the 
ventricles is interrupted by the large horse-shoe-shaped auri- 
culo-ventricular opening that lies in the middle of the dorsal 
surface of the heart (PI. 5, figs. 1 and 2). The margins of this 
aperture sweep round on either side to meet the conspicuous 
auriculo-ventricular plug posteriorly. Externally there is no 
indication of any division of the ventricular part of the heart 
into two compartments. 
Attached, and immediately anterior to the posterior margin 
of the auriculo-ventricular opening, lying over the aperture 
— really in it — is a prominent, rather button-like structure of 
cartilaginous consistency — the auriculo-ventricular plug (PI. 
5, figs. 1 and 2, A.V.^l.). This structure, being in front of 
the posterior margin of the auriculo-ventricular opening, is 
enclosed by the auricular walls, and, owing to the posterior 
