628 



FIBRES OF THE HEART. 



layer arises chiefly from the pulmonary artery, 

 PP ; it first forms the pulmonary channel, P, 

 and then expands into a layer which crosses 

 obliquely over the apicial channel, associated 

 with fibres derived from one of the carneae 

 columnae. The basial portion of this layer 

 which crosses over the auricular channel, can- 

 not often be separated from the fibres of its 

 superjacent band, the fibres of the musculi 

 pectinati being intricately interwoven with 

 them. When this layer is replaced, its lower 

 loose edge applies itself to the anterior boun- 

 dary, a b, of this cavity, and is lined with its 

 internal proper membrane. Of the three layers 

 composing the proper wall of this ventricle, 

 two, the middle and inner layers^, are confined 

 at the edge of the septum, forming thereby the 

 lateral boundary of this cavity. 



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 

 fibres winding and reflecting upon themselves, 

 as shewn in jig. 281. The basial part of this 

 boundary, a*6, 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 Jig. 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 CPCAA A, 

 Jig. 282, with a fasciculus of fibres, c, emerging 

 from the middle layer of the septum, and with 

 another fasciculus of fibres, A, jig. 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. In 

 jig. 281 the internal layer of fibres, PC, is seen 

 crossing the cavity obliquely towards the apicial 

 part of the posterior boundary, and in jig- 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 

 carneae columnae, 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 Jig. 278 ; and 

 the radiated fibres wind round the axis closely 

 upon them, as is seen in Jig. 280. By this 

 radiation, instead of all the fibres passing 

 longitudinally, 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 it 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 arteriosus ; 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 



