THE DEVELOPMENT OF THE HEART IN MAN. 
289 
on the wall of the atrium, especially orally, forming the septum spurium, better 
termed the “ tensor valvulse.” 
The right venous valve grows rapidly forwards, and the slit-like sinu-atrial orifice 
enlarges orally and caudally, reaching to the floor of the atrium. The right valve 
extends forwards on the floor as far as to the atrio-ventricular cushions. It extends 
orally along the roof of the atrium and forms an almost complete partition, shutting 
off the portion of the atrium lying to its right side from the portion on the left. The 
prolongation of this venous valve extends to the left side of the atrio-ventricular 
orifice. 
The wall of the atrium to the right of this partial septum is smooth internally, 
but at 12 ’5 mm. muscle bands are present, and these increase in number and in size 
and constitute the musculi pectinati of the atrium. Up to a late stage of develop- 
ment these bands do not spring from the venous valve, but arise some short distance 
from it. They run obliquely down to the region of the atrio-ventricular junction. 
The further development of the portion of the atrium lying to the right of the 
right venous valve need not be more fully described. Its size varies in different 
specimens, depending largely upon whether the atrium is in systole or diastole at 
the time of fixation. 
The views of the external surface of the various hearts show how, on both the 
right and left sides of the heart, the marginal portions of the atrium expand and 
form two wing-like processes lying -on either side of the cylindrical bulbus cordis. 
In the adult heart they form the auricles or appendices of the atria. 
The most important and most complex developmental changes occur in the portion 
of the atrium lying to the left side of the right venous valve. 
The extension forwards of the right — and later of the left — venous valves into the 
cavity of the atrium and the septum primum atriorum mark off clearly a chamber to 
which it seems advisable to give a special name, since it can be recognised through 
all the later stages of development, and in the adult heart forms the “sinus venarum.” 
To this chamber of the embryonic heart I propose to give the name of the sinu- 
atrial chamber. Its limits are : on the right, the right venous valve ; on the left, the 
left venous valve and the septum primum ; the roof formed by the wall of the atrium 
between the upper diverging limbs, and the floor formed by the area between the 
lower attachments of the venous valves. In the adult heart this chamber includes 
the whole of the posterior part of the right atrium from the vena cava superior to 
the vena cava inferior, its right margin formed by the crista terminalis, and its con- 
tinuation the valve of the vena cava inferior, the left wall formed by the fossa ovalis 
and the limbus above it as far as to the roof of the atrium. The left wall is a com- 
posite wall, formed, as will be shown, by the septum primum in the floor of the fossa 
ovalis, by left venous valves merged into it, and by a partial fusion of the adjacent 
walls of the two atria. The lower attachment of the right venous valve to the floor of 
the atrium becomes modified after the 20-mm. stage. 
TRANS. ROY. SOC. EDIN., VOL. LII, PART II (NO. 12). 
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