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the placental Mammalia, relates to the very brief period during which the auricles intercommunicate in the Marsupials, and to the minute size, and in other respects incompletely developed state, at which the young marsupial animal respires air by the lungs, and has the mature condition of the pulmonary circulation established. The right and left auricles intercommunicate by an oblique fissure in the uterine embryo of the Kangaroo when two-thirds of the period of gestation is past, but every trace of

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this foetal structure is obliterated in the subsequent growth of the heart; so that in the mature animal the wide terminal orifice of the postcaval, ib. d, is separated from that of the right precaval, ib. b, by a simple crescentic ridge, ib. e, which forms a salient angle of the parietes of the auricle between these apertures. The orifice of the left precaval, ib. c, is close to that of the postcaval, in a position analogous to that of the coronary vein in Man, which here opens into the left precaval. The right auriculo-ventricular valve is membranous, and its free margin is attached by fine 'chorda tendineæ' to three mammillary columnæ carneæ;' these

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in the Kangaroo, fig. 401, arise from the septum of the ventricles, but in the Wombat, fig. 402, the base of two of the columnæ ' is situated at the angle between the septum and the thin outer wall of the ventricle. The right ventricle extends nearly to the apex of the heart in the Wombat; but falls short of that part in the Kangaroo. The ventricle is continued in a conical form, somewhat resembling a bulbus arteriosus,' to the origin of the pulmonary artery, f, figs. 401 and 402, and projects beyond the general surface of the

heart further than in ordinary Mammalia. The appendix of the left auricle is notched in the Kangaroo to receive the apex of this process, but not in the Wombat. Two pulmonary veins, i, fig. 403, terminate close together, or by a single trunk, at the upper and dextral angle of this auricle. The mitral valve is regulated by two short and thick mammillary columnæ, ib. k, k, which send their tendinous chords to the margin and ventricular surface of the valve.

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403

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Heart of the Wombat,

The ventricles and auricles present the usual Mammalian proportions and relative thickness of the parietes. Three sigmoid valves are situated at the origin of the pulmonary artery, and the same number at that of the aorta.

B. Heart of Lissencephala.-In most species of this subclass the right auricle shows the modifications resulting from the return of the blood thereto, as in Lyencephala, by two distinct precavals, of which the left opens alongside the postcaval into the lower (sacral) part of the auricle, as in figs. 401, 402. In the Porcupine a large Eustachian' fold is on the auricular side of the

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Capromys is an exception, among the Rodents: at least in the specimen I dissected, the blood from the head and fore-limbs entered the auricle by a single precaval vein. cxxx". p. 72.

postcaval aperture, and a slight ridge indicates the remains of the upper fold, forming the boundary of the sinus venosus.' In the great Anteater I observed that the resemblance to the auricular valve in Reptiles was rather closer:-the entry of the postcaval was guarded as usual by the Eustachian valve, or homologue of the lower of the two semilunar valves between the sinus and the auricle in the Crocodile (vol. i. fig. 339): and here there was also a narrower valvular fold or ridge on the opposite side of the postcaval orifice, answering to the second valve (ib.): a ridge is continued from both valves toward the opening of the precaval. In the Elephant, also, which shows its rodent affinity in the two precavals, there is, besides the Eustachian' between the orifices of the postcaval and left precaval, a remnant of the upper valve extending from the posterior side of the orifice of the right precaval.

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The inner surface of the right ventricle is smooth and even, little broken by trabeculæ, in Rodents and other Lissencephala. Two or three slender mammillary columns' send tendinous chords to the tricuspid valve in the Porcupine and Hare. The apex of the heart is sub-bifid in the Hare and Acouchi: it is simple and obtuse, with the ventricles broader and rather flattened from before backward, in the Beaver: it is relatively longer and less obtuse in the Water-vole: in neither of the aquatic Rodents are the foramen ovale or ductus arteriosus kept patent. In most Rodents the right ventricle reaches to the apex: in Helamys it even descends lower than the left ventricle. The heart is short and obtuse in the Sloths: the auricles almost cover the basal part of the ventricles: the pericardium adheres to the diaphragm by loose cellular tissue, and the thoracic part of the postcaval is short. The pericardium is not so attached in the Armadillos, and the heart is more oblong in shape, with the apex more sinistrad the lower third forming the apex is due wholly to the left ventricle, from the basal part of which the right ventricle projects, like an appendage, in Dasypus Peba. Orycteropus has the Eustachian, but not the Thebesian, valve: the muscular walls of the left ventricle are four times thicker than those of the right; but are almost smooth internally. With an unusual thoracic convexity of the diaphragm, in the Mole, is associated a less symmetrical position of the heart than in other Lissencephalans. The tenuity of the pericardium is a characteristic of many Insectivora: notably of the Hedgehog.

c. Heart of Cetacea. In these marine and fish-like Mammals the heart, like the brain, shows higher characters than in the preceding subclasses. The pericardium extends down upon cxxII. tom. iv. p. 486.

the abdominal muscles to reach the diaphragm, which has a like low position anteriorly, to which it adheres broadly and the precavals unite and terminate in the auricle by one orifice: the thoracic part of the postcaval is very short. The musculi pectinati are well developed in the right auricle, and the appendix is distinct, but undivided. The fossa ovalis is feebly marked in the Cachalot, is deeper in some Delphinidæ, but in all Cetacea it is closed: there are neither Eustachian nor coronary valves. In the Cachalots and Whales the ventricular mass is subdepressed and semicircular, the apex being rounded or rather flattened, and sometimes indented: for the right ventricle is coextensive with and sometimes terminates, as in the Mammalian embryo, distinctly from the left. In Phocæna and most Delphinida, the apex of the ventricle is simple and better marked. The movable wall of the right ventricle has about half the thickness of that of the left, showing the exercise of greater force in propelling the blood through the lung, than in land Mammals. The tendons of the tricuspid valve go to three short and thick columns in most Cetacea; but the rest of the inner surface is broken by strong trabecular bands. Hunter notes the soft yielding substance of the semilunar valves in the Hyperoödon he dissected, suggesting that they were naturally less strong than in land Mammals. The left auricle is less than the right, with many well-defined muscular columns on the inner surface, and a distinct appendix ; but is less fleshy than the right auricle. In the left ventricle both trabecular and mammillary forms of muscular processes of the inner surface are numerous.

The most striking feature in the anatomy of Whales is the vast size of their several organs: the heart may be more than a yard in transverse diameter, and not much less in length.

D. Heart of Sirenia.-The outward division of the ventricles indicated in some Cetacea

404

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Heart of the Dugong

is carried to an extent very characteristic of the present group: but in Rhytina and Manatus the cleft is not quite so deep as in the heart of Halicore, fig. 404.

1 CCXXVI. ii. p. 111.

In half-grown specimens of Dugong' I found the foramen ovale completely closed, and the ductus arteriosus reduced to a thick ligamentous chord, permeable only for a short distance by an eye-probe from the aorta, where a crescentic slit still represented the original communication. In the smoothness and evenness of their exterior, and their general form, the auricles of the Dugong, ib. a, d, resemble those of the Turtle (Chelone, vol. i. fig. 335): the appendix can hardly be said to exist in either. The right auricle, a, is but little larger than the left, e: the musculi pectinati are well developed, especially in the left: they are irregularly branched, and with many of the small round fasciculi attached only by their two extremities to the auricular parietes. There is but one precaval and one postcaval orifice in the right auricle, with a smaller coronary inlet. The pulmonary veins terminate in the left auricle by a common trunk one inch in length. The free wall of the right ventricle scarcely exceeds at any part a line in thickness, and is in many places even less. The tricuspid valve is attached to three fleshy columns by chordæ tendineæ given off from the sides and not the extremities of those columns, both of which extremities are implanted, as trabeculæ, in the walls of the ventricles. There are several other columnæ carneæ passing freely from one part of the ventricle to another, like the musculi pectinati of the auricles, and which have no connection with the tricuspid valve. The mitral valve is adjusted to its office by attachments to two short and transversely extended mammillary columnæ. The thickness of the parietes of the left ventricle varies from half an inch to an inch. The valves at the origins of the great arteries, c, ƒ, present the usual structure.

E. Heart of Ungulata.—In all hoofed beasts the ventricles are conical; the apex being longer and sharper in Ruminants than in most other Mammals. The auricles are relatively smaller to the ventricles than in the preceding groups. The three parts of the tricuspid valves are distinct from their confluent bases, and are pointed at the apex: the basal union of the two parts of the mitral valve is of a greater extent, forming there an annular valve about the left auriculo-ventricular opening. The smooth inner surface of the ventricles is but little interrupted by fleshy columns. The Horse resembles the Ruminant in the general shape and structure of the heart: but in the Tapir2 it is shorter and broader, as it is in the Rhinoceros3 and Elephant. The right auricle in the Rhinoceros, as in most Ungulates, has but one precaval orifice, and shows no valve at the termination of 1 cxvII". p. 35. 3 v'. p. 46.

2 CLII".

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