480 
DR. PETTIGREW ON THE ARRANGEMENT OE THE FIBRES 
and posteriorly, and form flattened rings, which accommodate themselves to the shape 
of the ventricle. 
The points in which the fibres of the right ventricle differ from those of the left are 
the following : — 
1st. They are more delicate. 
2ndly. They form segments of spirals and flattened rings, instead of double conical 
spirals. 
3rdly. The external fibres enter the anterior coronary groove to become internal, not 
at one particular point, as in the apex of the left ventricle, but throughout its entire 
extent, the broad band at the base excepted. 
4thly. The fibres of the right ventricle form a constriction anteriorly (the so-called 
fleshy pons). This constriction separates the pulmonary artery from the auriculo-ven- 
tricular opening, and does not exist in the left ventricle, unless the septal segment of 
the bicuspid valve is taken to represent it. 
Vertical section of the right ventricle {Mammal). 
When a vertical section of the right ventricle is made posteriorly (Plate XIV. figs. 
43 & 44, s), it is found to taper in two directions, as in the left. It differs, however, 
from a similar section of the left ventricular wall in being comparatively much thicker 
towards the apex. This arises from the manner in which the right apex is formed, and 
is readily explained according to the segmentary process which is believed to separate the 
right apex from the left in utero. Into the extreme apex of the left or typical ventricle, 
as was shown, only one layer enters ; whereas into successive portions of the left ventri- 
cular wall (at slight removes from the extreme apex) two, three, and four layers enter. 
But the right apex is known to be separated from the right side of the left apex at a 
part considerably above its extreme point, and where the left ventricular wall is some- 
what thickened ; so that probably two or even three layers enter into the construction 
of the right apex. 
Transverse sections of the right and left ventricles {Mammal). 
What has been said with reference to the difference in direction of the several exter- 
nal and internal layers of the right and left ventricles and septum, the common nature 
of the fibres posteriorly, and their independent nature anteriorly, the varying thickness 
of the right and left ventricular and septal walls, the conical shape of the right and left 
ventricular cavities, &c., is fully borne out by transverse sections. When a transverse 
section of the ventricles of the deer is made half an inch or so from the base (Plate XV. 
fig. 49), the following phenomena are observed : — 
1st. The cut ends of the more vertical fibres exteriorly {cdd') and interiorly {d d'd") 
are nearly equal in number, and are seen to the outside and inside of the deeper or 
more central fibres. The prevailing direction of the central fibres, on the septum {d) 
and left ventricular wall {e), is more or less circular, owing probably to many of the 
