472 
DR. PETTIGREW ON THE ARRANGEMENT OE THE FIBRES 
able partition or septum which shall occlude the right auriculo-ventricular opening during 
the systole. The manner in which the several loops act is determined by their direc- 
tion. Thus the more vertical ones, in virtue of their contracting from above down- 
wards, have the effect of flattening or opening out the valvular fold, and in this way 
cause its dependent or free margin to approach the septum. The slightly oblique fibres, 
which contract partially from above downwards, but principally from before backwards, 
assist in this movement by diminishing the size of the right auriculo-ventricular orifice 
in an antero-posterior direction, —it remaining for the very oblique and transverse fibres, 
which contract from before backwards, and from without inwards, to complete the move- 
ment, by pressing the inner leaf of the fold directly against the septum — an act in which 
the blood plays an important part, from its position within the valve, this fluid, according 
to hydrostatic principles, distending equally in all directions and acting more immedi- 
ately on the dependent or free margin of the valve, which is very thin and remarkably 
flexible. When a vertical section of the fold forming the valve of the bird is made, 
that portion of it which hangs free in the cavity is found to be somewhat conical in 
shape, the thickest part being directed towards the base, where it has to resist the 
greatest amount of pressure — the thinnest corresponding to its dependent and free 
margin, where it is applied to, and supported by, the septum. The upper border of the 
fold is finely rounded, and in this respect resembles the convex border which limits the 
right ventricle of the mammal towards the base. 
The spindle-shaped muscular band (Plate XIV. fig. 40, h ), which from its connexion 
may be said to command the upper (e") and lower (j) portions of the right ventricle 
interiorly, is obviously for the purpose of coordinating the movements of the muscular 
valvular fold ; and as its position and direction nearly correspond with the position and 
direction of the musculus papillaris situated on the right ventricular wall of the mammal 
(Plate XIV. fig. 44, h!), it is more than probable that it forms the homologue of this 
structure. Indeed this seems almost certain from the fact that, if the ventricles of the 
bird be opened anteriorly (Plate XIV. fig. 40), and the band referred to contrasted with 
the anterior musculus papillaris of the left ventricle (y), both are found to occupy a 
similar position. The fleshy band therefore may be said to be to the muscular valve of 
the right ventricle, what the anterior musculus papillaris and its chordae tendineae are 
to the segments of the mitral valve. Compared with the tricuspid valve of the mammal, 
the muscular valve of the right ventricle of the bird is of great strength. As, more- 
over, it applies itself with unerring precision to the septum, which is slightly prominent 
in its course, its efficiency is commensurate with its strength. The prominence on the 
septum alluded to is very slight, and might escape observation, were it not that immedi- 
ately below it the septum is hollowed out to form a spiral groove of large dimensions (d 1 e"). 
This groove, like the valve, runs in a spiral direction from behind forwards, and from 
below upwards, and, when the valve is applied to the septum during the systole, con 
verts the right ventricular cavity into a spiral tunnel, through which the blood is forced, 
on its way to the pulmonary artery. Such of the fibres of the superficial or first external 
