It occasionally happens that the accessory fibres which arise from the interior of the right ventricle are not very numerous; in such cases a greater number of fibres arise abruptly from its surface.
The superficial layer has three sets of ori gins: one, primitive, from the interior of the left ventricle; the others, accessory, from the interior of the right ventricle, and from the outer surface of both. It cannot with pro priety be considered as one common invest ment, since each ventricle for the most part gives birth to its own superficial fibres. It is necessary to raise it as a distinct layer for two reasons : first, that the superficial fibres of the right ventricle in general pass nearly at right angles with their subjacent fibres, and there fore require to be removed in order to proceed with the next stage of separation : secondly, that it developes the peculiar mode of closing the left ventricle, and of forming the apex ; and probably no other method than that of the twisting of the fibres could have been so secure, especially as the parietes at the apex of the ventricle do not generally, even in a bullock's heart, exceed a tenth of an inch in thickness.
The second stage —The external layer having been traced from its origins to its insertions, we may now trace the deep-seated layers; and as these have, for the most part, the same origins, courses, and insertions as the super ficial layer, we may commence the description at the same points.
It has been already stated that the fibres of the rope and of the two carnea' eolumnw ex pand in a fan-like manner, that their inmost fibres pass through the apex and become ex ternal, but that the chief of them wind round the axis of the left ventricle above the apex, as exemplified in fig. 270, enc. The respec tive sets of fibres pertaining to these three bodies continue separate during their radiation only, after which they become plaited together by folding one over the others. Their mode of association is shown in the extended portion of the split layer, CRC in fig. 280, also in its counterpart, cite, winding round the api cial part of the ventricle. Again, in fig. 278, it may be seen that the fibres at the bases of these columns turn under and pass up in con junction with those of the rope forming the middle mass, cac, at the upper of which they fold over making flat twists upon them selves, which have, however, become exag gerated in appearance by the unwinding of the heart, as in rolling it up again some of the angles are converted into spires, preserving a considerable degree of parallelism.
Having shown the origins, and the method adopted in the association, of the fibres form ing the middle mass infig.278, we proceed by tracing the divisions and prolongations of its fibres, and the plan of building up the two chambers of the heart. First, the forma tion of the left ventricle. If the right carnea columna, cc, be replaced in contact with is fellow, and if the rope, RR, be brought round the upper part of this cavity so as to embrace them, and if portion 4 be split from the middle mass, cnc, and be wound, in association with the apieial fibres, CRC, round the lower part of this cavity, that division of the heart, comprising the left ventricle and the middle mass, will bear a near resemblance to that represented in ,fig. 280; in which figure the rope, RR, in embracing the heads of the carne:: column:', cc, brings into view its fan like fibres, rt, sweeping round the upper part of the axis of this ventricle ; in which the fibres of portion 4, in winding round the lower half of the axis, embrace the bodies of the carneve column:', cc, and associate with the apicial fibres, CRC, and in which the ex tended layer, cite, represents the middle mass minus the portion 4, which is split from it. Thus much pertains exclusively to the descrip tion of the formation of the left ventricle. That of the right is more complicated, and constitutes The third stage. In pursuing the mass of blended fibres, cac, occupying the middle of fig. 278, it is found that, after having formed the left, it splits under the line marked by stars into two bands, which embrace and con tribute to form the right ventricle. These sepa rated bands were stated in the preliminary remarks to be of unequal lengths, the longer making two and the shorter making but one spiral circle round the heart. The longer, in the first place, assumes the character of a layer and forms the middle layer of the septum. It requires to be described in three portions. Portion 1, being attached to the valve of the other section of the aorta, was stripped off in unwinding the heart; in the wound-up state it passes over the pulmonary channel of fibres, r, along the part marked 1, in its way to the aorta, AA; its absence, however, opens to view the fibres coming from the base and form ing the right layer of the septum. Portion 2 proceeds from the starred line across to enter Into the formation of the rope, an, and will be noticed hereafter. Portion 3 is the longer band; it is not entirely seen, being overlapped by some of the fibres of portion 4; it passes across to the niche, CPC, where it was di vided in unwinding the heart, in order to liberate the two ventricles which were encircled together by this band. Previously to pursuing this band further, it is better to trace it as the middle layer of the septum in its natural situ ation—the wound-up state of the heart. In fig. 280 it forms the extended layer, cac, in association with portion 2, and split from por tion 4, which does not belong to the septum ; on being replaced, its cut edge, a, applies to the cut edge, b, in passing as the middle layer between the right and left layers of the septum.
The middle layer is seen in fig. 282 emerging at the posterior edge of the septum, where portion 2 disconnects itself to join at the under surface the band above, but in this figure is marked C large, indicating that it is derived from this layer, which has hitherto been lettered CRC. This layer, being now deprived of all its other portions, will hereafter be considered as a band, and it has already been explained why it should be denominated the longer band. This band in emerging at the posterior edge of the septum is joined by another band of fibres, which is seen in fig. 281, forming part of the internal layer of the proper wall of the right ventricle; its fibres, Pc, arise from the pul monary artery, re, and from one of the carnea columns not in sight; they cross ob liquely over this cavity to the posterior edge of the septum to join the band in ques tion. By the intimate blending of the fibres of these two bands the apicial half of the posterior boundary of this ventricle is constructed. The longer band, now aug mented, is lettered accordingly in fig. 282, Cec, and in proceeding soon receives at its inner surface an accession of fibres, A, coming down from the aorta. This band, C: CA, in winding spirally from left to right round the left ventricle along its middle third, gradually approaches both the base and the surface : for when it arrives at the anterior edge of the sep tum it becomes the basial band, and having been traced round the left under the right ven tricle, in making its second circle it passes over that cavity. In fig. 281 the commencement of its second course is exhibited. It is bisected, one portion, Cm**, being held up by a probe; the other, at the anterior coronary track, a c t, receives at its inner surface a fasciculus of fibres, A, from the aorta, AA, and is also lettered CPCAA. This fasciculus and por tion of the band form together a groove, by winding over the pulmonary channel when brought down into its place, and which toge ther form the basial part of the anterior boun dary of this cavity. This hand in its progress round this ventricle constitutes the basial band of the middle layer of its proper wall, and forms so many connexions with the base, that to trace them all would be found a very com plicated piece of dissection ; it is, therefore, deemed better to give a general description of them. For instance, the aorta presents three different aspects under which this band is con nected to it: the first, at the termination of the anterior coronary track ; the second, between the pulmonary artery and the annulus venosus ; and the third, between the annulus venosus and the annulus arteriosus, or at the extremity of the posterior coronary track. The aorta re ceives at each of theseparts an insertion of fibres from the outer surface of the band ; and the band receives on its inner surface a fasci culus from the aorta. These reciprocal com munications occasion the band to be very firmly bound down to the base, and to be arranged, to a certain extent, into festoons. For each of these accessions from the aorta, an addi tional A is added to the lettering of the band, which is, accordingly, designated CreA AA A. As the band passes the annulus venosus, its outer fibres by a gentle obliquity in their course successively arrive at its tendinous margin, into which they become inserted immediately below those of the superficial layer, and some proceeding still more deeply pass under the tendinous margin into the ven tricle, and form the musculi pectinati. In order to avoid repetition it may be here remarked, that this part of the description ap plies to the annulus arteriosus also. The last two accessions of fibres this band receives should be traced, since they assist in the con struction of theposterior boundary of the right ventricle. In fig. 282 this band is seen in the latter part of its course round the right ventri cle, marked Cr( AAA; on reaching the pos terior coronary track, pet, it is joined on its inner surface by two fasciculi which bind it down to the base, but on each side of this track it is separated and raised. One of these fasciculi, the last derived from the aorta, is not seen in this figure; the other appears emer ging from under this ventricle, being portion 2 of the middle layer of the septum, which disconnected itself from this band, CPC, in its first circle round the left ventricle ; it is marked C large, being derived from the middle mass of fibres, cac, in fig. 278, in which portion 2 is seen crossing over to join the band CPCAAAAC, just before it becomes the rope ; the fasciculus of fibres A from the aorta A A is also seen joining this band at its inner sur face nearer the base. By the union of these two fasciculi with the band in question, the basial half of the posterior boundary of the right ventricle is formed. By pursuing, in fig. 282, this band or combination of fibres, lettered CPCAAAAC, it is seen to form, while it is gradually twisting upon itself, the brim of the left ventricle, and then to make a sharp twist of its fibres into the rope a a, by which means they are rendered the internal fibres of the left ventricle; in fig. 280 they may be traced expanding again into a layer, pursuing the same spiral sweep from left to right, but from the base towards the apex, and inwardly instead of outwardly. Thus the de monstration brings us back to nur starting-point.