:

G. Heart of Quadrumana.-In the Aye-aye, as in other Lemuride, the heart is rounded, subdepressed, with a very obtuse apex; much resembling that of the four-months fœtus in Man: the right auricle is much larger than the left it receives the blood by a single precaval, by the postcaval and coronary veins. There are both Eustachian and Thebesian valves, and a wellmarked fossa and annulus ovalis. These also characterise the right auricle in higher Quadrumana. The carneæ columnæ and chordæ tendineæ are more numerous in the right ventricle of Monkeys and Baboons, relatively, than in Man: the divisions of the tricuspid terminate in a broad and rounded margin; that next the orifice of the pulmonary artery being, as usual, the largest. In the left ventricle the columnæ carneæ are numerous and small, giving a strongly reticulate character to the inner surface.

The pericardium, which has a limited adhesion to the diaphragm, opposite the apex of the heart, in Lemurs, progressively becomes less perpendicular in the thorax as the Quadrumana rise in the scale, with concomitant shortness of the thoracic post-cava, and increasing extent of adhesion of the pericardium to the diaphragm but in none is the heart so broad at the base, so flattened, or so extensively supported by the diaphragm, as in Man.

H. Heart of Bimana.-In the prone trunk of quadrupeds the pericardium adheres to the sternum, rarely to the diaphragm; in erect bipeds the connections are reversed: no Mammal has so large a proportion of the heart resting upon the diaphragm as Man, where the central aponeurosis is concomitantly expanded for the attachment of the intervening part of the pericardium. Here the heart lies obliquely, not, as in most Mammals, parallel with the mesial plane: the apex, less acute than in Ruminants, and less obtuse than in aquatic Mammals, is directed downward, forward, and to the left, notching the anterior margin of the left lung, and beating across the interval between the cartilages of the fifth and sixth left ribs. The appendix of the right auricle has one undivided apex, extending over the origin of the aorta to that of the pulmonary artery. The single precaval terminates at the upper part of the auricle on a plane anterior to that of the postcaval, which is at the lower part: from the anterior margin of this orifice is continued the valvular fold called Eustachian,' which is often reduced in substance to a filmy network, or may be wanting between the postcaval orifice and that leading to the ventricle is the opening of the coronary vein, with its valve: above the Eustachian valve is the depression, fossa ovalis,' indicative of the closed oval intercommunicating vacuity in the

septum of the foetal auricles; bounded above by the prominent crescentic border, or annulus ovalis.' The opening into the ventricle is bordered by a sclerous oval ring, to which muscular fibres of both auricle and ventricle are attached; the ring being thicker for the latter.

In the Human right ventricle the portion of the tricuspid valve nearest the orifice of the pulmonary artery is the largest, and is divided by deeper notches from the two smaller portions than these are from each other: the chordæ tendineæ from each columna carnea are inserted, generally into the contiguous borders of two portions of the valve: the muscular prominences of the inner surface of the ventricle have either their inner or central surfaces free, or are free in the circumference of their middle part but attached at both ends, like beams (trabecula), or they project freely in a conical form, as 'columnæ mammillares: they are least developed in the conical prolongation of the cavity, (infundibulum, conus arteriosus), from the apex of which the pulmonary artery arises. The arterial orifice of the ventricle is formed by

sclerous tissue, which a dissector may define as a ring, fig. 405, disposed in three crescentic curves, with the convexities, a, a, toward the ventricle, and the blended horns, d, b, projecting toward the artery: the ring is represented as cut through at one of these points of confluence, e, e, in order to its being spread out. Muscular fibres of the right ventricle, f, f, are attached to the convexities of the ring; the fibrous coat of the artery is attached to the outer margin, the sigmoid valves, fig. 406, a, a, to the inner margin, of the upper or arterial surface of the concavities which owe their definition to the junction of the endocardium to such valvular attachments. The right ventricle continues to show, in Man as in other Mammals, the same relation, as an appendage to the left, which is illustrated in the section of the Bird's heart, vol. ii., fig. 92, forming, as so seen, a concave parabolic section of a cone, applied to the more perfect cone of the left

ventricle but the walls are relatively thicker to those of the left ventricle than in Birds.

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407

The left auricle, figs. 408 and 409, LA, lies to the left and back part of the base of the heart, is transversely oblong and subquadrate behind; its auricular appendage comes forward into view curving to the right, upon the base of the pulmonary artery. The walls of the sinus venosus' are thicker than those in the right auricle: the terminal orifices of the pulmonary veins, usually one on each side, sometimes two on the right and one on the left, are undefended by valves: on the septum, the foetal foramen is feebly indicated by a crescentic depression. The opening into the left ventricle is smaller than the right auriculoventricular one: it is defended by the pair of triangular folds of endocardium, called the bicuspid' or 'mitral' valve. Of these the largest, fig. 407, a, hangs between the auricular and aortic orifices, and is in part reflected from the sclerous ring of the latter: a small fold commonly also projects at each angle of junction of the larger folds. The chief conical 'columnæ' are two in number, and larger than those of the right ventricle; their apices are shown at fig. 407, P, p, each contributing tendinous cords to the portion of the mitral valve, a. The distribution of the chorda tendineæ, from each column to contiguous borders of the two parts of the mitral, obviously illustrates the adaptation to bring those margins together in the contraction of the ventricle. The semilunar valves at the aortic orifice, ib. d, are thicker than those of the pulmonary artery, the Valsalval sinuses,' e, are deeper, and the corpora arantii' larger: the muscular walls of the left ventricle are about three times thicker than those of the right: some of the inner longitudinal fibres, ib. b, are attached to that part of the aortic ring, not preoccupied by the larger mitral fold, a. The left ventricle is longer and narrower than the right and alone forms the apex: the two large mammillary columns occupy the lower three-fourths of the cavity, rising in its axis: the fibres radiate from their base and wind round the axis, being progressively

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sent off, so that few reach the apex of the column: but the moulding of the ventricle about these is not the cause of the conical figure of the heart, since this obtains where no such mammillary columns are present. (Vol. i. figs. 334-340.)

The heart is lined by a membrane, endocardium,' rather thicker and more opaque in the left than in the right cavities, especially in the auricle: thinnest on the muscular projections, both pectinate and columnate. The chief layer consists of a close network of elastic fibres, lined by a stratum of polygonal epithelial scales, constituting the free surface; and attached by an areolar tissue to the muscular coat. This is covered by the reflected serous layer of the heart-bag, called 'ectocardium."

The disposition of the intervening muscular fibres has been best illustrated in relation to the human heart. Those of the auricles

form a superficial layer,

fig. 408, common to both cavities, and also a deep layer, fig. 409, proper to Beach.

The superficial layer includes the transverse band of fibres, fig. 408, D, expanding as it passes to the right, RA, and left, LA, auricles. The deeper fibres

appear at the parts not covered by the superficial ones. Some, II,. arising from the annulus aorticus,' K, K, arch over the auricle, beneath D, contributing some fibres to the septum, at s: other arched orlooped' fibres, F, curve over the auricles and are attached by both extremities to the auriculo-ventricular rings AV and AA: a third series, C, surround the auricular appendages, AA, and encircle the terminations of the superior, cs, and inferior, CI, vena cava.

The winding or convolute disposition of these so-called 'annular fibres' is exemplified in fig. 409. The superficial and deep-seated fibres are, however, continuous, at parts of their course those marked E, fig. 408, of the former series, wind round the left auricle LA, and are continuous, with some intervening attachment to the aortic root K, with the ascending band F: a posterior band is shown at G, fig. 409, passing over the left auricle LA, and along the posterior border of the appendix A: some of the fibres, on reaching the anterior border, quit the band G, to join the fibres d forming the apex: other detachments from the band g, encompass the terminations of the pulmonary

409

veins, P, p. Like the muscular fibres of the tongue, those of the heart are not visibly connected together by areolar tissue; such connective medium, in the degree in which it may exist, can only be inferred through the help to unravelling gained by boiling the heart. The more obvious mode of connection is, as in the tongue, by reciprocal decussation or interlocking. LA In the ventricles the longer external fibres, e. g., wind upward round the apex and bend downward from the auricular and arterial rings at the

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base, to become internal, and so inclose, and, at the same time. contribute to form, the shorter, interposed loops; these, likewise, having similar relations to the layers of fibres which they successively inclose.

sively narrowed, or shown by smaller tracts of conformable course of fibres, as these are removed in dissection from without inward.

In the superficial ventricular layer they have a subspiral course, descending, in the fore part of the ventricles, fig. 410, to the left, and on the back

410

Superficial ventricular fibres; front view. CLXXXVII".

part to the right, being partially interrupted at the interventricular grooves, of which the anterior is shown at d. Those which cross the groove bridge over the coronary vessels; those which penetrate it curve upward and contribute to the right layer of the septum, and so help to encompass the right ventricle. The super