IN THE YENTEICLES OE THE VERTEBRATE HEART. 
451 
may corroborate by a reference to the accompanying Plates, engraved from photograph s 
taken by myself from the dissections. 
I. By exercising due care, I have ascertained that the fibres constituting the ventricles 
are rolled upon each other in such a manner as readily admits of their being separated 
by dissection into layers or strata, the fibres of each layer being characterized by having 
a different direction. 
II. These layers, owing to the difference in the direction of their fibres, are well 
marked, and according to my finding, are seven in number — viz. three external, a fourth 
or central, and three internal. 
III. There is a gradational sequence in the direction of the fibres constituting the 
layers, whereby they are made gradually to change their course from a nearly vertical 
direction to a horizontal or transverse one, and from the transverse direction, back again 
to a nearly vertical one. Thus, in dissecting the ventricles from without inwards, the 
fibres of the first layer, which run in a spiral direction from left to right downwards, are 
more vertical than those of the second layer, the second than those of the third, the 
third than those of the fourth — the fibres of the fourth layer having a transverse direc- 
tion, and running at nearly right angles to those of the first layer. Passing the fourth 
layer, which occupies a central position in the ventricular walls and forms the boundary 
between the external and internal layers, the order of arrangement is reversed, and the 
fibres of the remaining layers, viz. five, six, and seven, gradually return in an opposite 
direction, and in an inverse order, to the same relation to the vertical as that maintained 
by the fibres of the first external layer. This remarkable change in the direction of the 
fibres constituting the several external and internal layers, which is observed to occur 
in all parts of the ventricular walls, whether they be viewed anteriorly, posteriorly, or 
septally, has in part been figured by Senac *, and imperfectly described by Reid f , but 
has not, so far as I am aware, been prominently brought forward by any one. A few 
arrows will illustrate the gradation in direction referred to. 
External layers proceeding from left to right downwards of the spectator (posterior aspect) . 
->■ Fourth or circular layer occupying a central position in the ventricular walls. 
Internal layers proceeding from right to left upwards of the spectator (posterior aspect). 
* Op. cit. tom. i. and Appendix to tom. ii. f Op. cit. p. 591. 
