IN THE YENTEICLES OF THE VERTEBRATE HEART. 
477 
ventricle. The fibres of the third layer (Plate XIII. fig. 24) are continuous with 
corresponding internal fibres at the base, and proceed from two sources — the one set 
from the posterior third of the septum and the posterior half and anterior third of the 
right and left auriculo-ventricular openings ( b /), the other from the remaining ante- 
rior portions of the auriculo-ventricular openings and the anterior two-thirds of the 
septum. 
Fourth or central layer of the right and left ventricles [Mammal). The fibres of the 
fourth layer (Plate XIII. fig. 27 ,fd), unlike the fibres of the other layers, run athwart 
the ventricles, or at right angles to an imaginary line drawn from the base to the apex. 
Their direction, which from this circumstance is more or less circular, is accounted for by 
the external fibres, which run in a spiral direction from left to right downwards, turning 
abruptly upon themselves in this layer (Plate XII. compare fg , d e, with k of fig. 4) 
to reverse their course and proceed in an opposite direction, viz. from right to left 
upwards (Plate XIII. fig. 27, p q). The fourth layer consequently forms the boundary 
between the external and internal layers in both ventricles* ; and when it is removed the 
order of arrangement is reversed: the fibres, instead of proceeding from left to right down- 
wards, becoming more and more oblique, proceed from right to left upwards, gradually 
returning to an imaginary vertical in an inverse order. The fibres of the right ventricle, 
it may be observed, pass through the several changes in direction referred to, more 
rapidly than those of the left; in other words, the fibres of the right ventricle, when 
the dissection is conducted from without inwards, change from the nearly vertical to 
the horizontal, and from the horizontal back again to the nearly vertical at compara- 
tively slight depths from the surface, an arrangement evidently occasioned by the greater 
tenuity of the right ventricular fibres. The difference in the depths at which the layers 
of the right and left ventricles are found, introduces important changes in the appear- 
ance presented by the ventricles at different stages of the dissection. Thus, when the left 
ventricular wall is half dissected through posteriorly (Plate XV. fig. 54), the right 
ventricular one is quite dissected awayf. I say posteriorly, because, as was explained, 
the left ventricular wall anteriorly is but little affected, owing to the manner in which 
the fibres common to both ventricles radiate from the latter. The fibres of the fourth 
layer proceed in flattened fascicular bands from the auriculo-ventricular orifices all round 
(Plate XIII. fig. 27, bl), and illustrate very well the comparative depths at which the 
layers of the right and left ventricles are found. On transverse section the outer half 
of the central layer of the left ventricle posteriorly is ascertained to be on the same 
level with the internal layers of the right ventricle. Such of the fibres of the fourth 
layer as are common to both ventricles proceed from left to right, and, having crossed 
the posterior coronary groove, curve round on the right ventricle until they reach the 
groove for the anterior coronary artery (Plate XIII. fig. 23, oo'), where they dip in (r) 
to traverse the septum (Plate XIII. fig. 28, g e) in an antero-posterior direction, and so 
* The fourth layer of the right ventricle is represented at Plate XIII. fig. 28, p q. 
t The right ventricle is only half the thickness of the left. 
MDCCCLXIV. 3 s 
