THE DEVELOPMENT OF THE HEART IN MAN. 
297 
6 ’5 mm. in length. In my specimen, however, there is no separation such as he 
describes between proximal and distal cushions. Between the cushions 1 and 3 of 
Tandler there lie two additional cushions, namely, the distal bulbar cushions 
2 and 4. Successive sections of the bulbus in my specimen show that these 
swellings are extremely small and indefinite, and, were it not for the fact that four 
such cushions are readily recognisable in the developing hearts of lower forms, it is 
extremely doubtful whether they would be described as occurring at all in the 
human heart. 
In short, it is extremely doubtful whether there are distal bulbar cushions as 
distinct from the large proximal cushions in the human embryo at all. 
The cushions described above pass as far distally as to the root of the aortico- 
pulmonic septum. 
The figures from reconstructions of this portion of the heart in embryos from 
6 to 12'5 mm. show the successive transitions which occur. 
Thus in the 6-mm. embryo the distal portion of the bulbus cordis forms a 
cylindrical tube, lying within the pericardium and obliquely crossing the upper part 
of the atrium (Plate-fig. 2). 
The muscle coat does not extend as far as to the pericardium, and beyond the 
muscle-coated part is the truncus arteriosus. At the junction of bulbus and truncus 
the right and left sixth arterial arches arise dorsally very close to one another. 
Within the bulbus there are two distinct and large endocardial cushions, which 
almost meet with one another, so as to divide the lumen into two channels. These 
channels form the exit from the heart for the blood from both the left and the right 
ventricles, and at this stage the whole of the blood from the left ventricle must pass 
through the narrow interventricular foramen. 
In the next specimen (Bl) the portion of the bulbus is more oblique in position and 
lies more horizontally on the roof of the atrium, and its muscle coat ceases at a greater 
distance from the pericardial wall. 
Within the terminal portion the lumen of the bulbus is divided into two 
channels — -one, ventral and to the left, is continuous with the orifice of the fused 
sixth arterial arches ; and the other, dorsal and to the right, is continued into the 
truncus arteriosus. 
Proximal to this region, within the bulbus, are two large, plump spirally arranged 
cushions, one of which passes to the interventricular septum, and the other to the 
right margin of the atrio-ventricular opening. 
In the succeeding specimen (Ml) the condition is very similar, and does not 
require special description. The figure shows that the large cushions are continued 
along the walls of the two separated vessels in the terminal part of the bulbus, and, 
in addition, some small intermediate swellings of a similar character have appeared 
there (bulbar cushions 2 and 4) (Plate-fig. 8). 
There is at this stage no definite formation of either aortic or pulmonary valves. 
TRANS. ROY. SOC. EDIN., VOL. LII, PART II (NO. 12). 47 
