The Muscular Fibres Of The Heart may be traced, first in the ventricles, and afterwards in the auricles. In order to prepare the heart for the examination of these fibres, it should be hardened by maceration in alcohol, or by boiling: its external and internal membranes may be then cautiously raised, and the different layers of muscular fibres examined, commencing with those most internal, and following carefully the course of the fibres.

* Physiological Anatomy, vol. i. p. 161.

First:—in each of the ventricles wo find a proper set of fibres arranged so as to form a small conical sac, open at both extremities, the inferior opening being much the smaller: these may be termed ventricular sacs. In addition to the proper fibres, the ventricles have also a common set, which cover and unite the proper ones, and inferiorly at the apex of the heart become inflected and penetrate the small apertures above mentioned, in the ventricular sacs, on the internal surface of which they are expanded. They have been represented as forming six sets of layers in the left ventricle, and three in the right; the fibres do not confine themselves to particular planes; but the planes mutually penetrate each other, and are moreover united by fibres reciprocally detached from one to the other. The superficial fibres proceed spirally from the base to the apex; those on the anterior surface incline from right to left, and those on the posterior surface from left to right. Having arrived at the apex of the heart, they are inflected, as already observed, towards its interior, and therefore present in this situation, when the pericardium has been carefully dissected off, the appearance of a star, the rays of which are not straight, but curved. The inflected superficial fibres enter the openings in the ventricular sacs, and therefore both ventricles may be penetrated at the apex of the heart, without dividing the fibres. In the interior of the ventricles, some of the inflected fibres ascend from the apex on the interior of the same wall upon which they had descended in passing downwards from the base; others ascend on the opposite wall; and a third set terminate in the Caracae columna3. Secondly:—in each of the auricles the proper fibres arise from the tendinous zones: on the left side Some of them assume a circular arrangement in the vicinity of the auriculo-ventricular openings, and numerous oblique bands proceed from the same origin in various directions: one passes between the appendix and left pulmonary veins; another fills the interval between the pulmonary veins of the right and left sides, and others between the pulmonary veins of the same side, forming a border for their orifices; independently of which, the orifices seem specially provided with proper sphincters. On the right side, the part of the auricle corresponding to the junction of the superior and inferior cavae has no muscular fibres except a small band on the right side of the orifice of the superior cava.

In the rest of the auricle we distinguish two principal muscular bands: one embracing, in a circular manner, the right auriculo-ventricular opening; and the other descending from the interval between the right auricular appendix and superior cava, to terminate on the right side of the inferior cava. Between these two bands the muscular fibres are arranged in a fasciculated manner, constituting the musculi pectinati.

The superficial fibres of the auricles constitute a thin layer passing transversely from one auricle to another, and arising from, and terminating in, the tendinous zones.

From the above account, it follows that the right and left sides of the heart may be separated from each other by the division of the common fibres, leaving the proper fibres uninjured. For this purpose an incision should be made with caution through the anterior fibres of the ventricles parallel to the anterior fissure of the heart, and then the right and left sacs, above described, constituted by the proper fibres, may be separated with the finger. In order to separate the auricles, the incision should be made parallel to their posterior median fissure, and still greater caution is necessary here than in the separation of the ventricles. The ventricular sacs have been described as having a conical form: this is strictly true, more particularly of the left side, all parts of the exterior of the left sac being convex; but on the right side the part of the sac which is applied to the left ventricle is concave. Now, the reverse occurs in the auricles, the right presenting a convexity which is received into the concavity of the left.

Mr. Searle remarks, that "the fibres of the heart are not connected together by cellular tissue, as are those of other muscles, but by an interlacement which in some parts is very intricate, and in others scarcely perceptible. At the entire boundary of the right ventricle they decussate, and become greatly intermixed; at the apex and base of the left ventricle they twist sharply round each other, and so become strongly embraced; but in general the interlacement is so slight that they appear to run in parallel lines. Whether a mere fasciculus, or a considerable mass of this last description of fibres, be split in the direction of the fibres, a number of delicate parallel fibres will present themselves, some being stretched across the bottom of the fissures, perfectly clean and free from any connecting medium whatever; and although some must necessarily be broken, yet these are so few that they do not attract attention unless sought for. The disposition of the fibres varies in different 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 internal proper membrane. The fasciculi contribute to the formation of the bands. By tracing the fibres in bands, we are enabled to develop the formation of the ventricles in a progressive and systematic manner. The bands spring from a mass of fibres which forms the apicial part (the apex) 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. The average width of the bands is not less than a third of the extent between the apex and base of the left ventricle. A considerable mass of fibres may be exposed winding just above the apex; at the septum it splits into two bands: the one, a 'short band,' encircles spirally both ventricles, one half round the right, the other half round the left ventricle. The second, or 'longer band,' describes two circles: it first passes through the septum, and round the left ventricle; it secondly passes round the base, and includes both ventricles in its circuit. The fibres of this band, in forming the brim of the left ventricle, make a sharp twist like those of a 'rope,' by which means they become the inner fibres of this chamber, and expand into a layer which enters largely into the formation of that mass which has already been described as forming the apex of the left ventricle and dividing into the two bands. Thus the principal band, although it receives several increments-of fibres, has no complete beginning nor ending, a considerable portion of it originating and terminating in itself.

" The septum of the ventricles is composed of three layers; a left, a middle, and a right layer. The two former properly belong to the left ventricle; and the last, or right layer, exclusively pertains to the right ventricle. The two former are composed of the primitive mass of fibres derived from the ' rope' already alluded to as forming the brim of the left ventricle, and the carneae columnae of the same ventricle. The last, or right layer of the septum, has not the same origin as the two former have; its fibres arise from the root and lower margin of the valve of that section of the aorta which pertains to the right ventricle, from that part of the root of the pulmonary artery contiguous to the aorta, and from the carneae columnae of the right surface of the septum".

" It appears from the patient and laborious investigations of Mr. Searle, that the great mass of the fibres of the heart are arranged in a spiral direction; that many of them take a single curve, so as to surround both ventricles; that others dip into the septum and form a double curve, one surrounding the right ventricle, the other the left; whilst several others penetrate from the exterior into the apex, and become continuous with the carneae columnae in the interior of the ventricles." *

The spiral course taken by the fibres of the ventricles, and the continuity of the external with the internal fibres of these cavities, were known long ago to Winslow, Lancisi, Lower, and Gerdy.