66 
JANE I. ROBERTSON. 
valve-like fold (PL 5^ fig. 2, P.f.). The auricles open into 
the ventricles round the margins of the auriculo-ventricular 
plug and there are no auriculo-ventricular valves apart from 
this (PL 5, figs. 1 and 2, A. V.pL); the ventricular opening is 
guarded and closed from the side of the ventricle by the 
auriculo-ventricular plug; the bnlbus cordis opens directly 
from the ventricle, and its aperture is undefended by any 
but the vestigial valves previously described (PL 5, figs. 1 
and 3, a, h, c). 
In the absence of direct physiological observations we may 
assume that the heart functions are as follows : venous blood 
from the sinus venosus and arterial blood from the pulmonary 
vein enters the right and left auricles respectively ; when the 
auricles contract the sinu-auricular opening will be closed, 
partly by the action of the valve guarding its right side and 
partly by the bulging of the slack right wall of the pulmonary 
vein into it (PL 5, fig. 1, P.f.), while regurgitation along the 
pulmonary vein will be prevented partly by the action of the 
margins of the pulmonary aperture itself and partly by the 
marked obliquity of the pulmonary vein immediately before 
entering the auricular cavity (PJ. 5, fig. 2, P. V.) ; the 
contents of the auricles will then be discharged into the 
ventricles on either side of the auriculo-ventricular plug. 
With the contraction of the ventricles and of the interventri- 
cular septum the auriculo-ventricular plug is drawn in a 
ventral direction, and this, with the contraction of the muscles 
encircling it, closes the auriculo-ventricular aperture; simul- 
taneously the proximal end of the bulbus is approximated to 
the interventricular septum, and the contents of either 
ventricle are discharged along the corresponding side of 
the .spiral valve of the bulbus. On the contraction of the 
proximal muscular part of the bulbus cordis the two blood- 
streams are guided on either side of the septum of the first 
part to the transverse portion ; in the transverse part of the 
bulbus the blood from the right side of the heart passes along 
the posterior channel behind the vertical septum, while the 
blood from the left side of the heart passes along the anterior 
