IN THE VENTRICLES OE THE VERTEBRATE HEART. 
459 
The fibres of the fourth layer, like those of the other layers, are composed of two sets 
( fg , dd'e), the one proceeding from the entire septum and a limited portion of the auriculo- 
ventricular opening on either side of the septum anteriorly and posteriorly, the other 
from the outer aspect of the auriculo-ventricular opening and its remaining anterior and 
posterior portions. Although the fibres of the fourth or circular layer form the boun- 
dary between the external and internal layers, they do not on this account occupy the 
centre of the ventricular wall, as may be seen on transverse section (Plate XV. fig. 50). 
On the contrary, the circular layer ( e ) is found considerably nearer the exterior (c) than 
the interior (d) of the ventricular wall — an arrangement which was to be expected, since 
the layers, as was stated, increase in thickness from without inwards. The aggregate 
of the external layers, or those to the outside of the circular layer, is consequently much 
less than the aggregate of the internal ones. The fibres of the fourth layer are from 
their position embraced by the fibres of all the other layers — viz. those of one and seven, 
two and six, and three and five. The amount of spiral made by them is rather less than 
one full turn (Plate XII. fig. 4 ,fg, d d! e). 
Internal layers of the left ventricle of the Mammal. 
Fifth layer. When the. three external and the fourth or central layer are removed, one 
is immediately struck with the change, or rather complete reversal, in the direction of 
the fibres. Thus the fibres of the fifth layer (Plate XII. fig. 5), which are the internal 
continuations or counterparts of those forming layer three, instead of proceeding in a 
spiral direction from base to apex and from left to right, proceed in a spiral direction 
from apex to base and from right to left ; the reason of which is obvious when it is 
recollected that the fibres of the fifth layer, when the layers are numbered from without 
inwards, form the first of the internal layers — i. e. of those layers found to the inside of 
the central layer, where, as has been explained, the order in the arrangement of the 
fibres is reversed. 
The fibres of the fifth layer, like the other layers described, consist of two sets (o, n) ; 
they, however, pursue a very oblique course, and in this respect resemble the fibres of 
the third layer, which they cross at a very obtuse angle. They moreover fold or double 
upon themselves in an outward direction at the base (Plate XII. compare n! with n" 
of fig. 5 *), and in so doing alter their direction (n"), and become continuous with the 
two sets of fibres forming the third external layer (Plate XII. fig. 3 ,fg, de). The 
fibres of the fifth layer are also continuous with the two sets of fibres forming the third 
layer at the apex (Plate XII. fig. 3, Jc). The amount of spiral made by them from the 
time they leave the apex until they reach the base, rather exceeds one full turn (Plate 
XII. fig. 5, o o', nn' n"). The fibres of the third and fifth layers embrace or overlap the 
fibres of the fourth or central layer, they themselves being embraced by the fibres of 
the first and seventh and the second and sixth layers. 
Sixth layer of the left ventricle {Mammal). The fibres of the sixth layer (Plate XII. 
* In this figure the internal fibres (n r ) are seen twisting or bending over in an outward direction to become 
continuous with the external fibres (n"). 
