464 
DR. PETTIGREW ON THE ARRANGEMENT OE THE FIBRES 
1st. It has been shown that the walls of the left ventricle, when the septum is 
included, are composed of four systems of spiral fibres, two external and two internal ; 
the external systems running from left to right downwards from base to apex; the 
internal systems from right to left upwards from apex to base. 
2ndly. That these spiral fibres are arranged in layers or strata, which increase in 
thickness from without inwards, and that the fibres composing them have each a differ- 
ent course, whereby they change their direction from the nearly vertical to the hori- 
zontal, and from the horizontal back again to the nearly vertical. 
3rdly. That the fibres composing the strata alluded to are as nearly as may be of the 
same length, and enter the apex and issue from the auriculo-ventricular orifice at the 
base in two distinct parcels or bundles. 
4thly. That the two sets of fibres forming the external layers are continuous at 
the apex and at the base with the two sets of fibres forming the internal layers, and 
give rise to twisted continuous loops, pointing to the apex and base respectively; 
the more superficial loops embracing in their convolutions the deeper or more central 
ones. 
5thly. That the apex is opened into, and the apicial and basal orifices (on account 
of the double cone formed by the ventricular cavity) widened, by the removal of such 
strata as are found to the outside of the central stratum. 
6thly. That the ventricular wall, like the ventricular cavity, tapers towards the apex 
and the base, the tapering towards the apex being very considerable, that towards the 
base being less appreciable. 
7 thly, and lastly. That the septum is of nearly the same thickness as the left ventri- 
cular wall, and must be dissected pari passu with it, if the left ventricle is to be con- 
sidered complete in itself. 
The right and left ventricles , septum , &e. considered relatively (Mammal). 
By far the simplest way to regard the right ventricle is, to consider it as a segment 
of the left one — a view which is favoured both by the actual structure, and all that is 
at present known of the foetal development of the organ. In works on embryology 
it is stated that at first the heart consists of a mass of nucleated cells ; that by and by 
it assumes the form of an elongated sac or dilated tube ; that about the fourth week a 
septum begins to arise up internally , which proceeds from the right side of the apex and 
anterior wall of the cavity, in the direction of the base, where the arterial bulb leads off ; 
and that about the eighth week this interventricular septum is complete. It is further 
stated that the walls of the ventricles are, comparatively speaking, very thick, the thick- 
ness of both being about the same ; but that on approaching the full period the left 
begins to be the thicker of the two — a change which was a priori to be expected, seeing 
that after birth the left ventricle has to perform nearly twice as much labour as the 
right. 
Beginning therefore with the left or typical ventricle constructed as described, it 
