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.

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

' cxx'. 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 Phocana and most Delphinidæ, 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

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 Dugong1 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, f, 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 cxvi". p. 35. 3 v". p. 46.

2 CLII".

either the postcaval or coronary veins: the contrast presented by the Elephant, in this respect, is significant. The strong chorda tendineæ of the tricuspid connect it, in most Ungulates, with three obtuse and transversely oblong carneæ columnæ: one rising from the movable wall, a second from the septum, and a third smaller one from the anterior interspace between the fixed and movable walls: the tendons diverge from each column to the two contiguous moieties of the divisions of the tricuspid-a provision ensuring the simultaneous action and outstretching of the three portions of the valve. Two smaller columns placed opposite to each other, one on the free, the other on the fixed wall, are connected in the Rhinoceros and many other Ungulates, by a single strong tendon passing across the cavity from the apex of one to the other.' In the Hog some of the tricuspid tendons pass to a thick short column' projecting from the free wall, others pass directly into the smooth convex fixed wall of the ventricle.

In most Ruminants, especially the larger kinds, there is a bent bone at the base of the heart, on the septal side of the origin of the aorta, and imbedded in the tendinous circle which gives attachment to muscular fibres of the ventricle; in the Giraffe this bone was two-thirds of an inch in length. Two such ossifications of the sclerous tissue have here been met with in Oxen and Red-deer: an ossified and an unossified piece of fibro-cartilage are more commonly observed: in the Horse these bodies at the septal side of the aortic ring are rarely ossified until extreme age.

F. Heart of Carnivora.-In the present group the heart is more obtuse at the apex, and the left ventricle forms a greater share thereof, than in Ungulates. The Eustachian valve is wanting in most Carnivora; where indicated, its remains have been found in the smaller kinds, as the Weasel, Polecat, Ichneumon, which by their size resemble the immature of the larger species. The inner surface of the ventricles, especially the right, is more fasciculated, and the number of carneæ columnæ is greater than in Ruminants. A condensation of the sclerous tissue of the aortic ring in the Lion and Tiger, at two points, indicates the homologues of the heart-bones in Ungulates. In these and other Felines the mammillary columns are continued from the septal end of a strong trabecular tract between the 'fixed' and 'free' walls of the right ventricle. The heart in Phocide is broad and somewhat flattened,

I have not found, in Ruminants, so exclusive an origin of the mammillary columns from the 'free' or external wall, as described in ccxxxix. t. I. p. 502, after CLXXXV".

with an obtuse apex: the appendix of the right auricle is bifid, one process covering the origin of the pulmonary artery, the other lying upon the right ventricle. The auricular septum seems to be formed by an extension of the left part of the wall of the anterior cava, terminating in an arch to the right of the postcaval orifice, which thus seems to open (as it did in the embryo) into the left auricle. In the younger of two Seals, (Phoca vitulina), which I dissected,' the valve that cuts off this original communication between the auricles was incomplete, and left a large foramen ovale:' in the older Seal, not full grown, the valvula foraminis ovalis' was complete as to its extent, and the margins were adherent, save at the upper part where an oblique aperture, admitting a goose-quill, remained. In a young Walrus,2 the entire margin of the valve was adherent, and there was no intercommunication between the right and left sides of the heart. A broad crescentic fold, looking downward, divides the sinus, or fossa, receiving the precaval vein from the larger and deeper one receiving the postcaval one: this fold answers to the upper border of the 'fossa ovalis 'in the human heart; there is no orifice in the 'fossa' communicating with the left auricle. There is a small semilunar valve at the coronary orifice, but no Eustachian valve. The appendix of the auricle, in Trichechus, extends in front of the base of the aorta as far as the pulmonary artery, gradually contracting to an obtuse point: in Cystophora proboscidea the auricular appendix is short, broad, and bifid; in both it is occupied by a reticular arrangement of carneæ columnæ. The ventricles are broader in proportion to their length, and the apex is not produced in Trichechus, as in Cystophora proboscidea: the tendinous cords of the anterior division of the tricuspid valve, and a few of those of the right or external division, are attached to a short and thick fleshy column from the free wall of the ventricle; this column is connected by a short and thick trabecula' with the septum: most of the other tendinous cords are attached to the septum, and a few to trabeculæ connecting that fixed wall with the free wall of the ventricle. The pulmonary artery presents no peculiarity; it is connected by the ligamentous remnant of the ' ductus arteriosus,' which is 10 lines long and 5 lines in diameter, to the under part of the aortic arch, just beyond the origin of the left subclavian; its cavity is obliterated, but a short, thick, semilunar fold of the lining membrane of the aorta, with its concavity turned toward the end of the arch, indicates the place of the former foetal communicating channel.