268 
PROFESSOR D. WATERSTON ON 
right extremity opens into the bulbus, has produced a corresponding alteration in 
the muscle coats, and the atrial muscle wall at the right side is now directly 
continued into the muscle tissue of the bulbus wall, from which it was primarily 
separated by the whole width of the base of the ventricle. 
Septum . — The interventricular septum springs from the floor and ventral wall of 
the ventricle, and extends on the dorsal wall as high up as to the endocardial 
cushions. 
The interventricular foramen is an almost circular opening lying between the 
apex of the muscular interventricular septum on the one side and the fused endo- 
cardial cushions on the other (Plate-fig. 7). The opening leads from the cavity of 
the left ventricle into that of the right at the junction of the conical and cylindrical 
portions. 
The arrangement of the tissues around the venous ostia is particularly instructive, 
for it shows tt 3se openings before any definite valve apparatus has been formed. 
The transverse width of the atrial canal measures rather less than 90 mm., while 
the atrium and the ventricle are more than twice this diameter. The canal is very 
short, forming an annular narrow constriction, and there the muscle walls of the 
atrium and ventricle are directly continuous with one another. 
The connective tissue of the annulus fibrosus is considerably thicker than on other 
parts of the heart, and on section it forms a large oval mass. 
The inner part of the wall of the left ventricle, beside the left venous ostium, 
forms a loose network, and is becoming undermined in the fashion described by His. 
At the right ostium a more definite lateral valve cusp is formed. Here the mass 
of cushion tissue formed by the downward prolongation of the bulbar cushion B forms 
a large oval swelling in the inner surface of the wall. This cushion extends nearly 
to the lowest part of the ventricle, and there it lies on a band of muscle tissue which 
has become undermined and passes to the right surface of the ventricular septum, 
like the “ moderator band” of the adult heart. 
There are no indications of medial cusps at either the right or the left orifice. 
The interventricular septum passes downwards with smooth walls from the medial 
border of each ostium. 
Nor are there any definite indications of the formation of papillary muscles, with 
the exception of the strand already referred to in the right ventricle. 
The arrangements and attachments of the endocardial cushions, which form the 
medial boundaries of the venous ostia, are shown in the linear reconstruction 
(text-fig. 5). 
Right Ventricle . — The figure of the external form at this stage (Plate-fig. 5, 
compared with Plate-fig. 2) shows the nature of the considerable changes which have 
taken place. 
(l) The conical basal portion or right ventricle proper has increased in its 
relative size. 
