m THE VENTRICLES OE THE VERTEBRATE HEART. 
467 
- — a portion nipped off as it were from the perfect cone. (Compare c"d"d with cdd' of 
fig. 50, Plate XV., and EF with H I of diag. 17, Plate XVI.) 
5thly. When casts of the interior of the ventricles are taken, the left ventricular 
cavity (Plate XII. fig. 17), in accordance with the more perfect arrangement of the 
fibres forming the left ventricle, supplies a highly symmetrical double conical screw, the 
right ventricular cavity (Plate XII. fig. 16), although it has the same twist, furnishing 
only an incomplete portion. 
6thly. When the so-called common fibres posteriorly (Plate XIII. fig. 21, ffi) are dis- 
sected, layer after layer, synchronously with the independent anterior fibres (Plate XIII. 
fig. 23, p q), both sets are seen to pass through the same changes in direction ; in other 
words, they proceed from left to right downwards, gradually becoming more and more 
oblique as the central or transverse layer is reached. 
7thly, and lastly. When the fibres of the septum (Plate XIII. figs. 19, 22, & 25, g e) 
are dissected, layer after layer, with the other portions of the ventricular walls (Plate 
XIII. figs. 18, 21, & 24, ffi), they are observed to pass through the same changes in 
direction ; i. e. they pursue a spiral course from left to right downwards, becoming more 
and more oblique as the central layer (Plate XIII. fig. 28, fg, d e) is reached, after 
which they reverse their course and become more and more vertical in an inverse order. 
They consist moreover of three kinds : first, such as, properly speaking, belong to the 
right ventricle (Plate XV. fig. 45, l ) ; secondly, such as belong more particularly to the 
left ventricle ( n ) ; and thirdly, such as belong partly to the one ventricle and partly to 
the other (Plate XV. fig. 50, e'). Thus, in dissecting the septum from the right side, 
the fibres first met with belong almost exclusively to the right ventricle. These fibres, 
if traced from below upwards like the other internal fibres of the right ventricle, proceed 
from right to left. Traced from above downwards, their direction is just the reverse, or 
from left to right ; and it is important to note this circumstance, as the internal fibres 
of the right ventricle become mixed up on the septum, at no great depth from its sur- 
face, with fibres belonging exclusively to the left ventricle *, the direction of which is 
also from left to right downwards. There is therefore a portion of the septum in 
which the internal fibres of the right ventricle are mingled with the external fibres of 
the left, and where the two sets pass through each other as the fingers of the one hand 
might be passed between those of the other. The fibres found still deeper, and which 
in fact constitute the left two-thirds of the septum, belong exclusively to the left 
ventricle. These points may be readily established by dissection. 
External lagers of the right and left ventricles ( Mammal ). 
Superficial or first external layer. If the ventricles are dissected together (Plate XIII. 
figs. 18 & 20), the fibres of the superficial or first external layer posteriorly (Plate XIII. 
* The existence of these fibres in the right third of the septum induced me, when describing the left 
ventricle, to regard the septum as forming a part of its walls. 
