The layers.—Although the heart admits of being split into a number of layers, yet there being no material division formed by fascia or condensed cellular membrane, such separations are strictly arbitrary. It is, however, found convenient to separate the fibres into certain layers, in order to give a methodical de monstration of the formation of this organ. The same remarks obtain regarding the bands.
It is generally supposed that the superficial fibres properly constitute a distinct layer, form ing a common sac, which encloses the two ventricles. This is not strictly the case, for it has the same origins and terminations as have the fibres immediately subjacent to it. Neverthe less, the superficial fibres are, in the following description, considered as a separate layer, to show the peculiar construction of the apex.
The rope.—It has already been stated that the longer of the two bands terminates at the base in the rope. The fibres of this band, in forming the brim of the left ventricle, make a sharp twist like those of a rope, by which means they become the inner fibres of this chamber, and expand into a layer which enters largely into the formation of the mass which divides into the two bands. So the principal band, although it receives several increments of fibres, has no complete beginning nor ending, a considerable portion of it originating and ter minating in itself, which circumstance renders it necessary to fix upon the most convenient part of its course for the commencement of the demonstration.
Although the system here adopted of unra velling the fibres of the heart be strictly arbi trary, as every other must be, yet it will, most probably, be found the only method by which all the various courses, and several connexions made by the fibres in forming the heart, could be displayed.
The demonstration.—It is requisite to pur sue two methods of demonstration ;—one, de scribing the dissection, or unfolding, which consists in unravelling and separating the fibres, and tracing, from the circumference to the cen tre of the heart, their various courses, in the form of bands, by which they become in order unwound, and by which a general view of the formation of the two ventricles is at the same time presented. The other, describing the formation, or winding up of the fibres, comprehends the retracing of the fibres from the centre to the circumference, showing their respective origins, associations, courses, con nexions, and terminations, also the manner in which they are wound up to form the two ven tricles into one compact conical body.
The dissection.—The first stage consists in separating the superficial fibres from the two ventricles, which, perhaps, cannot be accom plished in a more simple manner than by rais ing them in the forms of two wings and a tail, as represented in fig. 279, which is to be done by commencing at the anterior coronary track, cutting through the superficial fibres and de taching them by means of a blunt scalpel in their natural direction, so far as their insertions at the base ; this will be found to divest the right ventricle, and, from their obliquity, a part of the left. (See the left wing, CAcc.) Then recommencing at the anterior coronary track, the fibres should be separated in the contrary direction, over the left ventricle towards the apex. These fibres take a very spiral course,
and as they approach the apex converge, but on reaching it they twist sharply round upon themselves, like the fibres of a thick cord, and entering at the apex become the internal fibres of this chamber. The remaining part of the superficial fibres, extending from the apex to the base, pertains exclusively to the left ventri cle; these should be divided an inch or two above the apex, and the apicial portion detach ed, which will complete the tail, CRC. Its fibres are represented, as they appear after sepa ration, untwisted. The basial portion of these fibres should now be detached so far as the annulus arteriosus, and reflected like the right wing, Cnc. These, as do most other fibres which approach the base, take a more longitu dinal course, and in general they become so separated as they diverge to encompass the basial part of the heart, that they cannot be raised in an entire layer unless some of the subjacent fibres be taken with them.
The second stage of the dissection comprises the disconnecting the bands which compose the outer or proper wall of the right ventricle. The superficial layer of fibres having been re moved, there remain two other layers pertain ing to this wall of the ventricle, viz. the middle and the internal. The middle is separable into two bands, the upper or basial, and the lower or apicial. It is better to detach the apicial band first, which makes one spiral circle round the heart. Its outer extremity being attached to the root of the aorta at its anterior face, and sometimes to the pulmonary artery also, an in cision should be made extending from the up per part of the anterior coronary track obliquely towards the annulus arteriosus, which incision should, in a calf's heart, be a little more than an inch in length and a tenth of an inch in depth. The band should then be detached agreeably to its spiral course from the base and middle third of the left, and from the lower half of the right ventricle, as far as the anterior co ronary track, the line from which the separation commenced. It here receives on its posterior surface a considerable accession of fibres from the right surface of the septum, by the junction of which this part of the boundary of the ven tricle is formed, but the further separation of the band prevented. In fig. 281, in the first or basial part of its course it is indistinctly seen, marked CACC. In,fig. 282 its middle course may be traced, although the half circle of the band which wound round the left ventricle has been cut off'. In the preparation exhibited in this figure the separation of this band could not be effected under the posterior coronary track, on account of the separation having been con ducted too deeply, where the fibres decussate to form theposterior boundary of the right ven tricle. In fig. 281, which exhibits a dissection of the right ventricle of a bullock's heart, the whole of the band, CACC, is separated as far as the anterior boundary of this cavity, and lies extended ; and the accession of fibres it re ceives from the right surface of the septum are seen prolonged into it.