456 
DR. PETTIGREW ON THE ARRANGEMENT OE THE FIBRES 
functional ; second, to obtain for the apex, which is the weak part of the ventricular 
wall, great strength with comparatively little material * ; and third, to procure for the 
fibres constituting the external and internal layers, which envelope all the others, a lati- 
tude and universality of motion which go far to account for the freedom and force with 
which the left ventricle contracts. 
The fibres of the superficial or first external layer arise, as a rule, from the fibrous 
ring surrounding the aorta (Plate XII. fig. 1, a, and Plate XY. fig. 46, a), and from the 
auriculo-ventricular tendinous ring (Plate XV. fig. 46, n). A few, however, are con- 
tinuous, beneath the auriculo-ventricular tendinous ring, with the fibres of the carneae 
columnse f (Plate XY. fig. 47, d). 
The fibres of the superficial or first external layer, with their internal continuations the 
fibres of the seventh or last internal layer, overlap and embrace the fibres of all the other 
layers, viz. those of two, three, four, five, and six, — an arrangement due, not, as was sup- 
posed, to the greater length of the fibres composing the first external and the last internal 
layers, but to the direction and position of the fibres of these layers on the ventricle — the 
fibres of the superficial or first external layer and those of the seventh or last internal one 
pursuing an almost vertical spiral direction on the body and wider portion of the ventricle, 
and appearing longer by twisting rapidly round the apex or narrow part, the fibres of the 
deeper layers pursuing a more oblique spiral direction on the body or wider portion of 
the ventricle, and appearing shorter from not reaching to the apex or narrow portion. 
Second external layer of the left ventricle {Mammal). The fibres of the second 
external layer (Plate XII. fig. 2), like those of the first, advance spirally from left to 
right downwards in two separate sets (fg, d d! e ), — the one set proceeding from rather 
more than the anterior half of the septum, and the anterior and inner portion of the 
auriculo-ventricular opening ( d ) ; the other from rather less than the posterior half 
of the septum, and the posterior and outer part of the opening (f). At the apex the 
fibres from the anterior portions of the opening and septum go to form the posterior 
half of the apicial orifice (Plate XII. figs. 2 & 11, Jc), where they become continuous 
with the anterior fibres of the sixth layer (Plate XII. fig. 6, nn"); while those from the 
posterior portions of the opening and septum enter into the formation of the anterior 
half of the apicial orifice (Plate XII. fig. 11, l), and become continuous with the posterior 
fibres of the sixth layer (Plate XII. fig. 6, o o'). I use the term apicial orifice, because 
the apex is opened into when the two sets of fibres which constitute the first layer 
* The apex in the ventricle of even the horse does not exceed the eighth of an inch in thickness. This 
deficiency in the thickness of the ventricular wall, which, during the dilatation of the organ and the first stage 
of contraction, ensures great freedom of motion, is prevented from operating injuriously during the latter stages 
of contraction, by such portions of the musculi papillares as are situated at the apex plaiting into each other so 
as entirely to obliterate the apicial cavity. 
f It is a great mistake to imagine that all the fibres of the ventricles arise from the auriculo-ventricular 
tendinous rings, the fact being that, with the exception of the fibres of the first and seventh layers, they are con- 
tinuous beneath them. These rings are more fully developed anteriorly than posteriorly, and the ring which 
belongs to the left ventricle is stronger than that which belongs to the right. 
