THE DEVELOPMENT OE THE HEART IN MAN. 
259 
ventricle are joined to one another by a short narrow channel of great importance 
in the later stages of development, termed the atrial canal. In describing these 
chambers the terms “ proximal ” and “ distal” may be used, the former indicating the 
sinus end and the latter the truncus arteriosus end of the heart. 
The stage at which the following description begins is that at which the heart has 
the form of a twisted tube, showing the constrictions and dilatations referred to above. 
It is convenient to begin the description of the successive stages by giving an 
account of the heart in the youngest specimen examined, an embryo of twenty-seven 
pairs of somites (2Wl) (l), maximum length (from sections) about 3 mm., from 
which models of the heart and trunk at a magnification of 100 and 200 diameters 
were made. 
The larger model of the heart of this embryo is shown in the Plate-fig. 1. Viewed 
from the front, a portion of the atrium, the atrial canal, the ventricle and “ bulbus 
cordis,” leading to the commencement of the truncus arteriosus, are visible. In its 
general shape the heart bears a resemblance to the model constructed and figured by 
P. Thompson from the R. Meyer embryo (No, 399) of eighteen pairs of somites (2). 
At this stage the heart tube is twisted so that the atrium lies oral and dorsal to 
the ventricle, the atrial canal lies on the left side, and the ventricle and “ bulbus ” 
are sharply bent upon one another. The ventricle lies on the left side and at the 
apex, and the “ bulbus” extends along the right margin and curves orally to the left. 
The heart tube has become divided into segments, viz. sinus venosus, atrium, 
ventricle, and bulbus cordis, separated from one another by constrictions, and the 
principal bendings have occurred at the constrictions separating these chambers. 
The sinus venosus and atrium are not sharply bent upon one another, but the atrial 
canal marks the position of a sharp bend, in an oblique plane, between atrium and 
ventricle. At the bulbo-ventricular constriction a bend has been formed in an almost 
vertical plane. The bulbus is slightly curved to the left, and another bend is found 
at the junction of bulbus and truncus arteriosus. 
The terms “ ventricle” and “ bulbus cordis” are used here following Tandler (3). 
The terms and the significance of the chambers which they denote are discussed later. 
Table I. 
Dimensions of Heart Model of 2W1. x 200. 
Vertical height 
Transverse width of atrium . 
Vertical height of atrium 
Sinu-a trial orifice, height 
Atrial canal, width . 
Ventricle, dorso- ventral width 
„ transverse width . 
,, thickness of wall . 
Bulbus cordis, thickness of wall 
150 mm. 
. 85 ,| 
. no „ 
. 50 „ 
15 „ 
• 70 „ 
. 60 „ 
• 13 „ 
( lower part 13 ,, 
t upper „ 6 „ 
