the boundary of the right ventricle.—It is true that every part of the internal surface of this chamber contributes in forming its boundary. But, as this cavity is formed chiefly by the splitting of the mass of fibres into layers and by their re-union, it is clear that unless the layers so separated were well secured at their points of junction, their separation would pro gressively increase, and the cavity enlarge to a fatal extent by the repeated dilatations to which it is subjected. The mode of union which secures this lateral boundary merits therefore particular notice. As the lateral boundary corresponds to the edge of the septum, it admits of the same division into anterior and posterior. The anterior boundary being formed by the splitting of the layers, and the posterior by their re-union, their respective modes of construction are not precisely similar. The anterior boun dary is principally formed by a certain set of tihres winding and reflecting upon themselves, as shewn in fig. 281. The basial part of this boundary, a b, is formed of fibres A, from the aorta A A, winding over the pulmonary channel of fibres P, in contributing to form the band CPCAA. The fibres of this channel also con tribute to form this part of the boundary, as is represented in fig. 278. The apicial part of this boundary is obviously constructed by the fibres ACC which form the right layer of the septum being prolonged into the extended band, which on being replaced occasions them to be doubled upon themselves in passing over the apicial channel in association with the fibres of this band.
The posterior boundary is constructed by the re-union of the fibres which pass in front of the cavity with others which pass behind it, and by the attachment of some of the fibres at the base to the aorta. The basial half of this boundary being formed by the conjunction of the under fibres of the basial band CPCAAA, fig. 282, with a fascicidus of fibres, c, emerging from the middle layer of the septum, and with another fasciculus of fibres, A, fig. 278, arising from the aorta, AA. That part of the boundary contiguous to the base is greatly strengthened by the outer fibres of the basial band being attached to the aorta at its posterior aspect. And the apicial half of the posterior boundary being formed by the conjunction of the prin cipal part of the internal layers of fibres which cross obliquely the cavity of the right ventricle with the chief part of the fibres of the middle layer of the septum as they emerge at its pos terior edge, where they freely decussate. 1 n fig. 281 the internal layer of fibres, pc, is seen crossing the cavity obliquely towards the apicial part of the posterior boundary, and in fig. 282 their conjunction with the fibres which emerge from the septum is seen forming a firm union. But the lateral boundary is rendered doubly secure by the curious circumstance of the coronary vessels, deeply penetrating the sub stance of the heart along the entire edge of the septum, stitching down, as it were, just on the outside of the boundary, all the fibres which form it.
The conical form of the heart.—The only point now remaining for consideration is the conical form of the heart. This form admits of the following explanation. Along the central cavity of the left ventricle are placed the two carnew columnre, the length of which is equal to the lower three-fourths of the length of the axis of this cavity. The fibres of these two bodies radiate, as represented in fig. 278 ; and the radiated fibres wind round the axis closely upon them, as is seen in fig. 280. By this radiation, instead of all the fibres passing longitudioally, which would have preserved these bodies in a state of equal thickness throughout their length, they are progressively parting with their fibres, retaining but a few, which, by their longitudinal course, reach the apex ; consequently these columns gradually diminish, becoming pyramidal, and forming together an inverted cone; and as the fibres in well-formed hearts wind closely round these columns, the entire ventricle gently assumes the form of a cone. And although the right ventricle is, as it were, appended to the left, yet at is not so connected to it as to destroy the conical form, but, on the contrary, in such a manner as to form a concave parabolic section of a cone which adapts itself to the gentle cone of the left ventricle. The two ventricles thus united assume the form of the more rapid cone of the heart.
Construction of the auricles.—For the pur pose of ascertaining the mode in which the fibres form the auricles, large hearts, as those of bullocks and horses, should be selected. Not withstanding the muscularity of the auricles is very much greater in large than in small hearts, yet the plan is the same in both, although less developed in the latter.
The fibres of the auricles arise chiefly from the tendinous margins of the annulus venosus and annulus artcriosus ; they ascend interiorly, and arrange themselves into several columns, which give off branches. Some of the branches form a simple communication between two of the trunk-columns, but most of them subramify, by which means the interstices are filled in. In small hearts the columns are not only more slender, but more numerous and in terlaced ; in these, the interstices in many places are not filled in, the internal and external proper membranes being in contact, and thus corn pleting the wall. Fig. 284 affords an interior view of a section of the right auricle, in which, the lining membrane being removed, the fibres are seen arising from the tendinous margin of the annulus venosus AV, forming the internal part of the wall of this auricle, and in their progress up arranged into columns, c, the branches of which are entwined together so as to construct the appendix. These convo luted columns at the posterior aspect of the appendices are flattened, as shown in lig. 285, c, where their fibres are associating together, and in passing round the edges to the anterior sur face become evenly arranged again,as seen in the appendix A of the right auricle, RA, of fig. 286. Thus far the construction of the two auricles agrees, the fibres of each arising from its respec tive annulus, forming first the inner part of the wall of the auricle, and then being arranged into columns which entwine together, forming the whole of the appendix. The fibres of the right auricle, after having formed the wall of this cavity, are prolonged to form the outer part of the wall of the left auricle. As may be seen in fig. 286, the fibres which extend from the con voluted fibres of the posterior surface of the right auricle, RA, wind evenly arranged, some over the apex, and others round the auricle, marked c, completing the outer part of the wall of the entire auricle : they then meet at the septum S, across which they pass associated together, marked D, and on reaching the left auricle divide into an upper portion and an anterior and posterior band. The upper portion is composed of the continued fibres D, which proceed up the appendix and encircle its apex. The anterior band E winds round the left. au ricle LA, and on reaching the root of the aorta K, its fibres become more or less at tached to it in different hearts ; in its course upwards, marked F, when it has completed a circle it passes behind the fibres which form the first part of the circle to enter into the formation of the fleshy columns of the appendix. The posterior band passes over the left auricle be tween the appendix A and the vena cava su perior cs ; and in fig. 285 it can be traced, coming over, marked 6, and passing along the posterior surface of this auricle LA, including in its course the posterior edge of the appendix • ; the fibres which pass along the posterior edge of the appendix, on arriving at the ante rior edge, separate from the band o to pursue their course round the edge of the apperelix,— now along the anterior edge,•and join the fibres D, which cap the apex. This division of the band which encircles the appendix is con stant, and evidently affords particular strength to its edge. The band itself o winds down towards the base, expanding and surrounding the orifices of the pulmonary veins P ; some of its fihres become lost on the surface of the auricle, and the others may be traced to the root of the aorta.
This band cannot be completely detached in consequence of some of its fibres being inter woven with its subjacent fibres.
The left auricle, without the addition of these bands, would nearly balance in substance and strength the right ; their addition gives, there fore, to the left a considerable preponderance in these respects over the right auricle.
The septum S is, in fig. 286, shown to:be com posed, superiorly, of the transverse band of fibres a, which passes from the right to the left auricle ; in its middle part, of the ascending fibres rr, which arise from the root of the aorta K, and pass up behind the baud a, some joining this band, the others proceeding to the vena eava superior es; and lastly, at the infe rior and posterior part, of a slender fitsciculus of fibres which crosses the septum transversely between the root of the aorta x and the vena cava inferior ci, extending from the annulus venosus to the left auricle, but which cannot be seen in this figure.
In concluding these remarks on the construc tion of the auricles, it may be mentioned that in the hearts of large animals a great difference ex ists in the structure of the two venre cavw, the superior being particularly fleshy, and the inferior apparently devoid of muscularity.