The absence of cellular substance as a con fleeting medium among the fibres in question is not only proved by the absence of its physi cal characters, but by its not being discovered through the medium of the microscope. Nei ther Mr. Kiernan nor Mr. Goadby, who exa mined the fibres with me, could detect its exist ence.
Since the chief utility of cellular membrane in investing and connecting together the fibres of a muscle is, most probably, that of retaining them within their proper spheres of action, and since the fibres of the heart are devoid of this agent, the question arises as to what other retaining power these possess. On this bead no difficulty presents itself; for the fibres, in winding round and round the cavity of the left ventricle, become arranged in concentric layers ; and in taking a larger sweep, in surrounding the right ventricle, the same arrangement is preserved, so that during the systole of the heart the whole mass of the fibres firmly com press each other, which necessarily retains them all within their proper spheres of action, ex cepting the superficial fibres, of which those towards the base, and especially those upon the right ventricle, where there is great latitude of motion, do not preserve a parallelism with their subjacent fibres, but lie nearly at right angles with them. It is on this account, most proba bly, that the superficial fibres have attracted notice, and have been viewed as a distinct layer.
The disposition of the fibres varies in diffe rent parts of the heart, forming parallel lines, angles, decussations, flat and spiral twists. The fibres are arranged in fasciculi, bands, layers, and a rope, which are so entwined together as to form the two chambers called the right and left ventricles. These are lined with their in ternal proper membrane.
The jitsciculi are connected with the aorta, pulmonary artery, and carnet: columns, and contribute to the formation of the bands.
The bands.—By tracing the fibres in hands, we are enabled to develop the formation of the ventricles in a progressive and systematic man ner. The bands spring from a mass of fibres which forms the apicial part of the left ventricle, and which, in winding round just above the apex of the heart, separates into two bands to form the right ventricle.
It will render the demonstration more intel ligible if a preliminary and cursory view be taken of the general course of these bands (fig. 283, p. 626,) by referring to the diagram. The bands, as there given, form a mere skeleton of the heart, merely indicating the se veral courses they take. The average width of the bands is not less than a third of the extent between the apex and base of the left ventricle. In the diagram, cite indicates the winding of a considerable mass of fibres just above the apex; at the septum, s, it splits into two bands. The shorter, CACC, encircles spirally both ventricles, one half round the right, the other round the left ventricle. The longer band de scribes two circles : it first passes through the septum, round the left ventricle marked CPCA ; it secondly passes round the base, and includes both ventricles in its circuit, marked progressively CPCAA, CPCA AA, CPCAA AA C, and It It.
After employing so many letters, it is requi site to explain that as the hands are frequently receiving fresh accessions of fibres, it is desira ble to characterise those increments individually by the initials of the names of the respective sources from which they are derived ; and in order to make a distinction between the indica tions of the fibres and of their respective origins, the latter are characterised by double, and the former by single initials. Accordingly, the aorta, the pulmonary artery, the rope, and the carnet' columns are designated AA, l'P, RR, and cc, while their fibres are marked A, P, n, and c. This plan is modified in one instance only, viz., the fibres of the main bulk of the heart, being derived from the rope and the two curlew columns of the left ventricle, are desig nated in the first instance by their proper ini tials CRC ; but as numerous increments of fibres are being made, in succession, to these three original sets, it is convenient to make an abbreviation in the lettering ; thus, etc is in dicated by C large, when combined with other initials; accordingly, CRCA is con tracted to CA, and CRCPCA to CPcA, and so with the rest.