458 
DE. PETTIGREW ON THE AERAN GEMENT QE THE EIBEES 
Third external layer of the left ventricle {Mammal). The fibres of the third external 
layer (Plate XII. fig. 3) resemble in their more important features the fibres of the 
second layer just described, and advance spirally from the base to the apex and from 
left to right in two distinct sets ( fg , d d! e ). They differ, however, from those of the 
second layer in their position on the ventricle. The set which enters more particularly 
into the formation of the anterior half of the apicial orifice proceeds from the posterior 
third of the septum, and the posterior half and anterior third of the auriculo-ventricular 
opening ; while that which enters into the formation of the posterior half of the apicial 
orifice (k) proceeds from the remaining anterior part of the auriculo-ventricular opening, 
and the anterior two-thirds of the septum. Arrived at the apex, which is now greatly 
widened, they bend or double upon themselves in a direction from without inwards, and 
reverse their course to enter the interior, where they become continuous with the two 
sets of fibres forming the fifth layer (Plate XII. fig. 5, o o', nn!). The fibres of the third 
layer differ from those of the second in being arranged in smooth fascicular bands, and 
in issuing from the auriculo-ventricular opening and entering the apex very obliquely 
— an arrangement which causes the fibres on the body of the ventricle (fdl) to pursue 
an almost transverse direction. They differ also from the fibres of the second layer, in 
not extending quite so far either towards the apex (Plate XII. compare k of fig. 3 with 
k of fig. 2) or the base (compare n! n" of fig. 5 with n' n" of fig. 6), and in being confined 
to more central portions of the ventricle — a circumstance which, as has been already 
explained, accounts for a vertical section of the ventricular wall tapering towards the 
apex and the base. The fibres of the third layer further differ from those of the second 
in exhibiting a less degree of crowding at the apex — for the very obvious reason that the 
apicial orifice, on account of the conical shape of the ventricle, becomes larger and larger 
with the removal of each successive layer. 
Fourth or central layer of the left ventricle {Mammal). When the three external layers 
have been removed, the fourth or central layer (Plate XII. fig. 4) is exposed. This 
layer may be denominated the circular layer, or layer of transition, from the fact that 
the fibres entering into its formation are circular, and form the boundary between the 
external and internal layers. It differs from the other layers as regards the quality and 
the direction of its fibres, and as regards its position in the ventricular wall — the fibres 
composing it being aggregated into strong fascicular bands, whose course is neither from 
left to right downwards as in the external layers, nor from right to left upwards as in 
the internal ones, but horizontal or transverse. This peculiarity in direction, which 
causes the fibres of the fourth layer {fg, d d' e) to run at nearly right angles to those 
of the first {j) and seventh layers (Plate XII. fig. 7, o n), is accounted for by the fact 
that in the fourth layer, the fibres of the third layer terminate or double upon them- 
selves, while the fibres of the fifth layer (Plate XII fig. 4, k) begin. In other words, 
the fourth layer, while it belongs neither to the third layer nor the fifth, forms the con- 
necting or transition link to both, as may be seen by a reference to Plate XII. fig. 4, 
where the fibres {d d' e) are seen to turn directly upon themselves {k). 
