282 
PROFESSOR D. WATERSTON ON 
thin laminae for a distance. of 25 mm., and between these two limbs is the rounded 
orifice of the coronary sinus. 
To the left of the right venous valve the vena cava inferior opens into the 
floor of the atrium, while the opening , of the vena cava superior is directed ver- 
tically downwards and about 7 5 mm. orally. The arrangement of the left venous 
valve and of the septum primum shows considerable modification towards the 
adult form. 
The left venous valve forms a small sickle-shaped ridge, 30 mm. in maximum 
height, projecting from the atrial wall especially dorsally and orally. It extends 
forwards on the roof of the atrium, and its anterior end merges into the right 
venous valve. 
The intersepto-valvular space has very largely disappeared, and the left surface 
Text-fig. 16. — Transverse section (slide B17) of the same heart as figs. 14 and 15. 
of the left venous valve is in contact partly with the right surface of the septum 
primum. The arrangement of these parts is shown in Plate-fig. 15. 
Two pulmonary veins, a right and a left, separated by a considerable interval, 
open into the left atrium. On the left side of the left vein is an enormous dilatation 
forming the cavity of the auricular portion. The ventricular orifice from the left 
atrium lies ventrally to the openings of the pulmonary veins. The opening into the 
right ventricle lies to the right of the right venous valve (text-fig. 16). 
Right Ventricle. — The cavity of the right ventricle is narrow but much elongated, 
and rudimentary valves are present around the right atrio-ventricular orifice in 
the form of a large muscular flap passing from the right margin of the atrio- 
ventricular junction towards the interventricular septum, while on the opposite side 
of the orifice there is a similar thinner cusp resting on the interventricular septum. 
The minute structure of the right cusp shows that it consists near its base of (l) a 
layer of muscle tissue continuous with the muscle tissue of the ventricle ; (2) a layer 
of embryonic connective tissue passing in a wedge-shaped manner from the atrio- 
