IN THE YENTEICLES OF THE VEETEBEATE HEAET. 
45' 
(Plate XII. fig. 11, fg), are removed. The fibres of the second layer are similarly 
arranged at the apex to those of the first ; i. e. they converge slightly and curve upon 
each other prior to doubling upon themselves to alter their direction and enter the 
interior. The fibres of the second layer differ from those of the first in being more 
fascicular, and in issuing from the auriculo-ventricular orifice and entering the apex 
more obliquely, the effect of which is to render their direction on the body of the 
ventricle slightly more transverse. They vary also somewhat from the fibres of the first 
layer in not quite extending either to the apex (Plate XII. compare Jc and l with f g 
of fig. 11) or the base (Plate XII. compare n! n" with q of fig. 6) — an arrangement 
which, as it also prevails in the deeper layers, satisfactorily accounts for the ventricular 
wall tapering towards the apex and the base respectively, as shown in a vertical section 
(Plate XII. figs. 13 & 14, s). The varying thickness of the ventricular wall towards the 
apex was well known to Geedy*, Duncan f, and Reid J, and wrongly, as I think, attri- 
buted by them to a supposed difference in the length of the fibres composing the different 
portions of it, rather than to the position and direction of the fibres themselves, which 
appear to me to afford the true explanation. The fibres of the second layer further 
differ from those of the first in their arrangement at the base, most of them being con- 
tinuous in this direction with the fibres of the sixth layer (Plate XII. fig. 6 , o n). The 
continuity of the fibres of the second external layer at the base with corresponding 
internal fibres, is strictly analogous to the continuity of the external fibres with the 
internal ones at the apex — the only difference being that at the apex the external fibres, 
in order to become internal, involute or turn in, whereas at the base the internal fibres, 
in order to become external, evolute or turn out. That the same principle which turns 
in the external fibres and secures their continuity with corresponding internal ones at 
the apex, also turns out the internal fibres and renders them continuous with correspond- 
ing external fibres at the base, is probable from the fact, that the borders formed by the 
union of the external with the internal fibres at the base and at the apex are convex, and 
identical as regards structure ; in other words, the fibres composing both borders advance 
spirally, the external fibres winding from above downwards and bending over the circular 
edge, forming the apicial orifice in a direction from without inwards (Plate XII. fig. 11, Jc l, 
and Plate XVI. fig. 55, ef ), the internal winding from below upwards and bending over 
the convex border surrounding the auriculo-ventricular or basal orifice in a direction 
from within outwards (Plate XII. fig. 7, n', and fig. 8, o"). The borders which limit the 
ventricle towards the apex and the base when the first layer is removed are consequently 
composed of loops or doublets of fibres (Plate XVI. fig. 55, ef, and Plate XV. fig. 47 d). 
Dr. John Reid§, in speaking of the left apex, says, “when the point is removed the 
circular edge is left entire, and is formed of another series of fibres, which, like those 
taken away, advance spirally from the base to the apex, and turning over the edge ascend 
in the opposite direction, continuing their course after being reflected.” The converse 
of these remarks holds true of the fibres at the base. 
* Op. cit. f Loc. cit. p. 591. 
i Loc. cit. p. 591. 
§ Loc. cit. p. 592. 
