FIBRES OP THE HEART. 



625 



Tht third stage. In pursuing the mass of 

 blended fibres, cue, occupying the middle 

 of Jig. 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, p, 

 along the part marked 1, in its way to the 

 aorta, A A ; its absence, however, opens to 

 view the fibres coining 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, RII, 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 

 Jig. 280 it forms the extended layer, cue, 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, 6, in passing as the middle layer 

 between the right and left layers of the septum. 

 The middle layer is seen in Jig. 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 Jig. 281, forming part of the 

 internal layer of the proper wall of the right 

 ventricle; its fibres, PC, arise from the pul- 

 monary artery, pp, and from one of the 

 carneae colurnnae 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 Jig. 282, 

 CPC, and in proceeding soon receives at its 

 inner surface an accession of fibres, A, coming 

 down from the aorta. This band, CPCA, in 

 winding spirally from left to right round the 

 left ventricle along its middle third, gradually 

 approaches both the base and the surface : for 



VOL. II. 



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 Jig. 281 the commencement of 

 its second course is exhibited. It is bisected, 

 one portion, CPC A A, 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 band 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 these parts 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 CPCA A A 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 the posterior boundary of the right 

 ventricle. In Jig. 282 this band is seen in the 

 latter part of its course round the right ventri- 

 cle, marked CP< 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, CRC, in fg. 278, in which 



2 T 



