476 
DR. PETTIGREW ON THE ARRANGEMENT OE THE FIBRES 
more than the anterior half of the septum and right anriculo- ventricular opening (l), 
and from the root of the pulmonary artery (k ) ; the other from rather less than the 
posterior half of the septum and right auriculo-ventricular opening (l), and from the 
posterior and outer half of the left auriculo-ventricular opening (b). The number of 
fibres, consequently, which proceed from the left ventricle (f) to cross the track of the 
posterior coronary artery (j), to become continuous with fibres having a similar direction 
on the right ventricle ( ff "), is greater in the second layer than in the first. 
Second layer of the septum — curious Y-shaped arrangement of the fibres— fibres of the 
right ventricle continuous anteriorly and posteriorly. The direction of the fibres of the 
second layer of the septum (Plate XIII. fig. 22, g) corresponds with the direction of 
the fibres of the second layer on the anterior and posterior aspects (ff, d d’) of the left 
ventricle, and with the direction of the fibres of the second layer of the right ventricle 
(Plate XIV. fig. 31 ,ff, d d'). In addition, the fibres of this and the succeeding layer 
diverge from each other at the upper third of the septum, and occasion an arrangement 
resembling the letter Y, the one portion bending over to assist in the formation of the 
fleshy pons anteriorly, the other, curving round to become continuous with the fibres of 
the right ventricle, having a like direction posteriorly (Plate XIY. fig. 31, fd). The 
fibres of the right ventricle (Plate XIV. fig. 35,/ d), as was shown, are continuous in the 
track for the anterior coronary artery (o) with fibres having a similar direction on the 
septum (g e’). They are likewise in many instances continuous with fibres from the sep- 
tum, in the track for the posterior coronary artery (Plate XIII. apply ge to f d’ of fig. 22), 
and form flattened rings. These points are best seen when the left ventricle is detached 
from the right, and the septum is dissected from the left side. 
Third external layer of the right and left ventricles (Mammal). The fibres of the third 
layer (Plate XIII. fig. 24) resemble in their course and general configuration the fibres 
of the second layer, and proceed in a spiral direction from left to right downwards 
anteriorly, posteriorly (ff, d 1 d"), and septally (Plate XIII. fig. 25, g e). They differ, 
however, from those of the second in being slightly more fascicular, in forming a thicker 
layer, and in having a direction which is almost transverse. The number of common 
fibres posteriorly (Plate XIII. fig. 24 ,ff", d 1 d") is greater in this layer than in the 
second, partly on account of their very oblique direction, and partly from their pro- 
ceeding from the left auriculo-ventricular opening nearly all round. Such of the fibres 
as cross the posterior coronary track to appear on the right ventricle* posteriorly (f d"), 
curve round in an anterior direction until they reach the track for the anterior coronary 
artery. Arrived here, they dip in at the anterior coronary groove (throughout its entire 
extent) to assist in forming the third layer of the septum (Plate XIII. fig. 25 ,ge). 
They also contribute to the Y-shaped arrangement of the fibres alluded to in layer two, 
the one arm or process giving off fibres to assist in building up the fleshy pons, the other 
giving off fibres to become continuous posteriorly with such as belong to the right 
* In this layer, as in the last, a certain number of the fibres do not cross the posterior coronary groove, but 
proceed at once to the left apex. 
