IN THE VENTRICLES OE THE VERTEBRATE HEART. 
487 
other), as was recommended when manipulating the single sheet. The additional sheet, 
which is thus made to pass through the same changes as the first, in nowise complicates 
the arrangement, and has the effect of doubling the layers, while it does not increase 
the number of spiral turns made by the lines composing them*. The object of having 
two sheets set at an angle is to ensure that the lines will enter the apex, and issue from 
the base, in two places, so as to render the cone bilaterally symmetrical, as it exists in 
the undissected left ventricle ; forit is found that, if only one sheet is employed, the 
apex of the cone, from its being composed of spiral lines, is lopsided, or like the barrel 
of a pen cut slantingly (Plate XVI. diagram 6, D). This difficulty is at once obviated 
by the employment of the second sheet ; for the one sheet being made by the angle of 
difference to wind from behind forwards (Plate XVI. diagram 10, C D), and the lines 
to enter the apex anteriorly (E), while the other winds from before backwards (A B), the 
lines entering the apex posteriorly (F), the apex, as every other portion of the cone, 
is rendered bilaterally symmetrical (Plate XVI. compare abe with c df of fig. 55 ; 
Plate XII. compare also fg and ed of figs. 9, 10, and xy of figs. 12, 13, & 14). When 
sheets adjusted as represented in Plate XV. diagram 2 are rolled up into a cone, as 
may be closely and conveniently imitated by placing the cone marked 5, Plate XVI., 
inside of that marked 4f , it will be found that a cone bilaterally symmetrical (Plate XVI. 
diagrams 7, 10, 11, 12 & 14), having a symmetrical apex (diagrams 7, 10, & 12, E F, 
diagram 11, XY, and diagram 14, DK) with walls consisting of seven layers, each 
having a different direction (Plate XVI. diagram 4, A, B, D, E, and diagram 5, B C D), 
is at once produced. Of these layers three are external, three internal, and one central, 
precisely as in the left ventricle itself (Plate XII. compare with figs, from 1 to 8 
inclusive). 
If the foregoing arrangement were reduced to a principle and applied to - the fibres of 
the left ventricle of the bird and mammal, it would be stated in something like the fol- 
lowing terms. 
By a simple process of involution and evolution , the external fibres become internal 
at the apex, and the internal ones external at the base ; so that whether they be traced 
from without inwards, or from within outwards, they always return to points not wide 
of those from which they started. The fourfold set of fibres, viz. the two external sets 
and the two internal, being spirally arranged round a cone, and running in two diame- 
trically opposite directions, it follows that, in order to involute and evolute, certain preli- 
minary changes are necessary. Thus the two sets of external fibres, which wind from 
the base of the cone to the apex in a direction from left to right downwards, make 
* Another effect produced by the two sheets being set at an angle and rolled within each other, which finds 
a counterpart in the ventricle itself, is the following. The lines composing the several anterior layers are rela- 
tively more vertical than those composing the posterior ones. 
f Strictly speaking the sheet composing the one cone should he rolled within that composing the other, so 
that, when the unwinding of the cones takes place, alternate coils or layers from either cone are removed in 
constant succession. 
3 t 2 
