478 
DR. PETTIGREW ON THE ARRANGEMENT OE THE FIBRES 
return to the posterior wall, where many of them are continuous with the fibres of the 
fourth layer of the right ventricle. The fibres which are not common, and which belong 
more particularly to the left ventricle, dip in at the posterior coronary groove to tra- 
verse the septum (Plate XV. fig. 54, g) in an opposite direction, or from behind for- 
wards, where they are continuous with the fibres of the left ventricular wall. 
Internal layers of the right ventricle {Mammal). 
When the fourth or central layer of the right ventricle (Plate XIII. fig. 23, p q), 
which is on the same level with the more superficial portions of the central layer of the 
left one (Plate XIII. fig. 27, fd), is removed, there is no longer any continuity between 
the fibres of the right and left ventricles posteriorly ; in other words, the common fibres, 
or those which pass from the one ventricle to the other, are dissected away, and the 
layers beyond or to the inside of the layer in question belong exclusively to one or other 
of the ventricles. This arrangement is apparently occasioned by the fibres of the poste- 
rior border of the septal duplicature (Plate XVI. diag. 16, G) passing through and be- 
coming blended with those of the posterior wall, only until the central layer in either 
ventricle is reached (Plate XVI. diag. 17, K). The internal layers of the right ventricle 
(Plate XIII. figs. 26 & 29, and Plate XIV. figs. 32, 33, 36, & 37), as has been partially 
explained, are, according to my belief, segmented portions of similar layers isolated from 
the left or typical ventricle by the primary notch or reduplication. This hypothesis is, 
I may observe, countenanced by their possessing the general characters of the internal 
layers of the left ventricle without being so complete. The fibres of the internal layers 
of the right ventricle proceed in a spiral direction from right to left upwards (Plate XIV. 
fig. 32, yp f , q q'), the fibres of the several layers, when the dissection is conducted from 
without inwards, becoming more and more vertical (Plate XIV. fig. 33, ypf, q q') as the 
interior is reached. They therefore pass through all the changes in direction through 
which those of the left ventricle pass — the fibres of the seventh or last internal layer 
crossing the- fibres of the superficial or first external layer at an acute angle, the fibres of 
the second and sixth layers at a slightly obtuse angle, and the fibres of the third 
(Plate XIV. fig. 31) and fifth (Plate XIV. fig. 32) layers at a very obtuse angle. When, 
however, the spirals formed by the fibres of the internal layers of the right ventricle 
(Plate XIV. figs. 36 & 37) are examined or compared with the spirals formed by the 
fibres of the internal layers of the left ventricle (Plate XII. figs. 6 & 7), the segmentary 
nature of the former is at once apparent, these in no instance forming complete double 
conical spirals similar to those found in the left ventricle, but only spiral fragments, or 
at most flattened rings, such as would be obtained by isolating or detaching a portion 
of a perfect cone composed of the double conical spirals described. 
Fifth layer of the right ventricle {Mammal). The fifth layer of the right ventricle 
(Plate XIV. fig. 32), from the fact of its being immediately to the inner side of the 
fourth central or transverse layer (Plate XIII. figs. 23 & 28, p q), is the first of the in- 
ternal ones (Plate XIV. fig. 32, ypf, q c[). The fibres composing it proceed in a very 
