446 
DR. PETTIGREW ON THE ARRANGEMENT OE THE FIBRES 
The fibres composing the ventricle of the fish consist of three layers — an external 
layer, a central transverse layer, and an internal one. The fibres of the external layer 
issue from the auriculo-ventricular opening and the opening for the bulbus arteriosus — 
some arising from the tendinous rings surrounding these apertures, others being con- 
tinuous with corresponding fibres in the interior. Their course on the base is from 
before backwards, and is more or less circular ; i. e. they flow in curves from either side 
of the auriculo-ventricular and arterial openings, towards the basal margin, over which 
they bend in graceful folds, to appear on the anterior and posterior borders and surfaces. 
On the borders, especially the posterior ones, they arrange themselves in parallel lines, 
and are continuous with each other at the angles and at the apex, where they are also 
continuous with the fibres of the internal layer. On the surfaces, the fibres of the 
superficial layer pursue a somewhat vertical direction, a certain number of them curving 
slightly upon themselves, and dipping beneath others having a more superficial position. 
The object of this arrangement is, to permit the superficial layer to furnish fibrous fila- 
ments which traverse the wall of the ventricle in a direction from without inwards, and 
which from this circumstance may be designated the perforating fibres. These perforating 
fibres connect the external and internal layers with each other, and with the fibres of 
the transverse or circular layer. Their function is obviously to approximate the various 
layers during the contraction of the ventricle, this' approximation being rendered neces- 
sary by the ventricular walls of the fish, as has been explained, being tunnelled in various 
directions by canals proceeding from the ventricular cavity — these requiring to be emptied 
of blood during the systole. 
When the external layer, which in the fish is comparatively thin, is removed, the 
transverse layer, with here and there the cut ends of the perforating fibres, is exposed. 
The fibres of the transverse layer are more or less circular, and differ from the fibres of 
the superficial layer in running at right angles to them ; they moreover occur in fasciculi 
arranged in parallel lines, and may be readily separated from each other. The trans- 
verse layer is of considerable thickness, and is connected with the external and internal 
ones by fibres which it gives off to, and receives from both. The appearance presented 
by the ventricle of the fish, when the transverse layer is taken away, is somewhat porous, 
owing to the ends of the perforating fibres, and the orifices of the canals which permeate 
the substance of the ventricle from within, being at this stage of the dissection exposed. 
On tracing the perforating fibres whose ruptured extremities are thus brought into view, 
their connexion with the fibres constituting the internal layer may be clearly made out. 
The fibres of the internal layer are continuous, in many instances, with the external 
fibres at the base, apex, and other portions of the ventricular wall, and are best exposed 
by cutting into the ventricular cavity, and dissecting from within outwards. They pro- 
ceed in well-marked fascicular bundles from apex to base, and resemble in their general 
character the carnese columnse of the ventricles of the bird and mammal. They differ, 
however, from the carnese columnse in question in having a more highly reticulated struc- 
ture. Situated within the reticulations, are a vast number of minute orifices commu- 
