IN THE VENTEICLES OE THE VEETEBEATE HEAET. 471 
fig. 39, g) to the fleshy pons anteriorly (e). The fold opens towards the interior of the 
ventricle (Plate XIV. fig. 40, i), in a direction from above downwards (Plate XIV. 
fig. 41, i), and is deepest at the edge of the septum posteriorly (Plate XIV . figs. 39 & 40, g). 
As it gradually narrows anteriorly (i), it is somewhat triangular in shape, its dependent 
and free margin (g) describing a spiral which winds from behind forwards, and from 
below upwards. The valve, from its substance and structure, may be appropriately 
termed the musculo-spiral valve, and is seen to advantage in the right ventricle of the 
emu (Plate XIV. fig. 41), swan (Plate XIV. figs. 38&39), turkey (Plate XIV. fig. 40), 
capercailzie, and eagle. It is composed of fibres from all parts of the floor and lower 
third of the right ventricle interiorly (Plate XIV. fig. 40, j), and from the upper third 
of the left ventricle and septum posteriorly (Plate XIV. figs. 39 & 40, g). The fibres 
from the lower third of the right ventricle interiorly, are spread over a large surface, 
and pursue a more or less vertical and slightly spiral direction. They gradually detach 
themselves in two portions (Plate XIV. fig. 40, h,j) from the right ventricular wall, and 
converge towards the centre of its middle third, where they form a flattened spindle- 
shaped muscular band (Plate XIV. fig. 40, h). Arrived at this point and continuing 
their spiral course, they diverge or spread out to assist in forming the inner and free 
leaf (i g) of the muscular fold (Plate XIV. figs. 39, 40, & 41, g) — one portion bending 
over in graceful spiral curves (Plate XIV. fig. 40, i) in a direction from within outwards 
and from below upwards, to become continuous with the superficial or external fibres at 
the base, a second portion bending over in like manner ( e ") to become continuous with 
certain fibres from the upper third of the second layer of the septum and left ventricle 
posteriorly, a third portion ( e "') pursuing a similar course to unite with the fibres from 
the upper third of the third layer of the septum and from the left ventricle. If that 
surface of the dependent or free leaf of the valve which is directed towards the right 
ventricular wall be examined, a fourth portion (g) is found to be continuous with the 
fibres of the upper third of the fourth or transverse layer. The muscular valve of the 
bird may therefore be said to be composed of the fibres entering into the formation of 
the several layers of the right ventricular wall, (the ventricular wall in fact bifurcates 
or splits up towards its base,) the external layers forming the outer wall of the valve, 
the internal layers, which are slightly modified, forming the inner. It is to this splitting 
up of the right ventricular wall towards the base (Plate XIV. fig. 41, Jc ) that its greater 
tenuity in this direction, as compared with the right ventricular wall of the mammal 
(Plate XV. fig. 49, y), is to be traced. If the muscular valve be regarded as an inde- 
pendent formation, which it can scarcely be, it will be best described as a structure 
composed of fibrous loops, these loops being of three kinds and directed towards the 
base — the first series consisting of spiral nearly vertical fibres forming a somewhat acute 
curve, the second series consisting of slightly oblique spiral fibres forming a larger or 
wider curve, and the third series consisting of still more oblique fibres and forming a 
still greater curve. As the fibres composing the different loops act directly upon each 
other during contraction, the object of the arrangement is obviously to supply a move- 
3 e 2 
