was

3800

1

12800

500

1

2500

4000

4

4800

by the size of the blood-discs.' In every individual a certain range of size was presented, and the two extremes and the average were recorded thus, in the Indian Elephant, the largest blood-discs were twice the size of the human, and the smallest was not less than 35th, the average being th of an inch. In the Chevrotain (Tragulus Kanchil) the average diameter of the blood-disc th inch. In the Giraffe the average size of the blooddiscs was 4th inch, or nearly one-third smaller than those of Man; the two extremes were 4th (few in number), th of an inch (more in number). The result of the examination of the blood of the largest of the ruminating tribe indicates that the size of the blood-discs relates to the condition of the whole organisation rather than to the bulk of the species. It would appear from the examination of the blood-discs in the goat, sheep, and ox, that an unusually small size of the blood-discs was associated with the peculiarities of the ruminant structure.' 3 This generalisation has not been affected by later observations. MANDL had discovered in the Dromedary that the blood-discs were elliptic. I confirmed the fact, giving the long diameter of the average-sized discs as th inch, the short diameter 5th inch; but I remarked that among the elliptical discs were a few of a circular form. Extending the observation to the smaller South American species of the aberrant ruminant family, I found the elliptical form to prevail in the blood-discs of both Llama and Vicugna.5 In the latter the average dimensions were, in long diameter 343, short diameter 19. Mr. Wharton Jones subsequently observed blood-discs of a circular form with the more numerous elliptic ones in the Llama. These exceptional instances to the Mammalian form of blood-disc are not associated with any other approximation to the oviparous type: the oval kind are equally non-nucleate with the ordinary circular blood-discs, and adhere to the ruminant characteristic of minuteness of size. Within the limits of that natural group, it will be observed that there is a ratio between the size of the blood-disc and that of the animal. But such ratio is quite inapplicable to the Mammalian class generally. If the Camelida repeat a reptilian shape of blood-disc, the Sloths have the largest blood-discs in proportion to the body: but neither one nor the other character occurs in the Monotremes and Marsupials which combine the greatest proportion of oviparous characteristics in their Mammalian organisation. In the Echidna and Ornithorhyn3 Ib. p. 284. • CLXXXI". p. 73.

86

1 CLXXIX".
CLXXXII", p. 1060.

2 Ib. p. 284.
5 Ib. p. 475.

3600

chus the blood-discs are circular and average 36th inch in diam. being larger in proportion than in Man, though less than in the Sloths. The numerous and insignificant gradations of size of Mammalian blood-discs between the two extremes noted in CLXXIX" have been recorded, decimally, in ccxXXIX, vol. i. p. 84.

§ 347. Heart of Mammalia.-In Mammals, as in other Hæmatothermals, the venous and arterial parts of the vascular system have no communication, beyond the heart, save at the peripheral capillaries.

The right auricle is less definitely divided into sinus' and auricle' proper than in Birds, and the intervening valves, always less efficient against reflux from the auricle into the sinus, gradually disappear. The right auriculo-ventricular valve resembles in structure the left, as being membranous and attached by tendinous threads to muscle. Other differences between the circulating systems of the two hot-blooded classes are shown by blood-vessels.

The heart, with its bag, or pericardium, is exclusively located in the thorax, and in many Mammals is more or less separated by a lobe of the lung, fig. 308, n, from the diaphragm, q.

A. Heart of Lyencephala.-In the Ornithorhynchus, fig. 308, a. b, c, it presents a rounded oblong, scarcely conical, form; it is situated in the middle of the fore part of the chest, parallel with the axis of the cavity, inclosed in a thin subtransparent but strong pericardium. The right auricle, b, is larger and longer than the left; its appendix is free and is slightly bifid. It receives the venous blood by three great veins; the left precaval, f, descending behind the left auricle, c, to join the termination of the postcaval, h; to the right of which the coronary vein also terminates in the auricle. The right precaval, e, is joined to the left by a transverse branch, g. There is a deep but closed fossa ovalis near the upper extremity of the septum of the auricles; indicating that the intra-uterine existence of the young was of longer duration than in the Marsupials. The right ventricle, a, is capacious, with thin parietes. The tricuspid valve consists of two membranous and two fleshy portions: the smallest of the latter is situated nearest the origin of the pulmonary artery, and corresponds with the lesser fleshy valve in the heart of certain Birds (vol. ii. p. 188, fig. 92, m): it is attached to the whole of the side of the first or adjoining membranous portion. The second fleshy portion answers to the larger muscular valve (ib. fig. 92, 1). The two edges of the lower half of the second fleshy portion of the valve in the Ornithorhynchus are free; but those of the

upper half are attached to the two membranous portions of the tricuspid valve; the margin of the membranous part of the valve is attached to the fixed wall of the ventricle by two small chorda tendineæ; and the structure of the valve thus offers an interesting transitional state between that of the Mammal and that of the Bird. The origin of the pulmonary artery is provided with the three usual sigmoid valves. The left ventricle has very thick parietes, which form the apex of the heart; the mitral valve is membranous; the larger flap is attached to two strong columnæ carneæ; the smaller flap also receives tendons from some smaller columnæ. The left auricle, c, receives two pulmonary veins. In the Echidna the free appendix of the right auricle is slightly indented. The terminal orifice of the right precaval is protected by a membranous semilunar valve, extending from its left side. The musculi pectinati diverge from a strong fasciculus which extends from the appendix to the orifice of the inferior cava; this fasciculus bounds the left side of a wide fossa ovalis, which is imperforate. The postcaval is protected by a large membranous Eustachian valve; the left precaval terminates by a distinct aperture to the left of the preceding, and is also defended by a process of the Eustachian valve. The inner surface of the right ventricle is more irregular than in the Ornithorhynchus; the free wall is attached to the fixed one by several columnæ carneæ and short chorda tendineæ: the tricuspid valve is membranous, and consists of one principal portion attached to the exterior circumference, and a smaller portion closing the outer angle; the free margin of the valve is attached to the extremity of a large fleshy column, arising by different roots from both the fixed and the free walls of the ventricle; a short fleshy column is attached to the left extremity of the valve; some chorda tendineæ are fixed to its right angle.

The heart of Marsupials offers no peculiarity in its general outward form. The apex is less obtuse in some species, as the Phalanger and Wombat, than in others, as the Kangaroo. The serous layer of the pericardium is reflected upon the large vessels near to the heart. The fibrous layer of the pericardium adheres to the sternum. The appendix of the right auricle is always divided into two angular processes, a, a, figs. 401 and 402, one in front and the other behind the trunk of the aorta, o. The right auricle presents the following marsupial conditions:- There is no trace of a fossa ovalis' or an annulus ovalis,' and the absence of these structures, which are present in the heart of all

1 xx. vol. ii. p. 52.

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

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

403

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.

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

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.