In the Ornithorhynchus, the spleen, fig. 308, u, u, is relatively large, and consists of two lobes bent upon each other at an acute angle: the anterior and right lobe is four inches long, the posterior and left lobe two inches and a half; the right lobe is bent upon itself. The artery of the pancreas is continued from the left end of that gland into the base of the spleen before its bifurcation. In the Echidna, besides the two lobes which are continued forwards from the left side, there is a third shorter descending appendage. The lobes are thin and moderately broad in both Monotremes. The Marsupialia repeat the bent or bilobed character of the spleen as shown in that appended to the left end of the stomach of the Phascogale, fig. 309. In the great Kangaroo (Macropus major) I found the main body of the spleen ten inches long, and the rectangular process six inches; both parts were narrow and thin.

In Lissencephala the spleen presents a more simple form, oblong, flattened, fig. 323, 1 (Rhynchocyon), with one end in contact, and having the usual vascular relation with the pancreas, ib. p. ib. p. The spleen is relatively longer and narrower in the Mole and Hedgehog: it is a thin elongate body, loosely suspended, in the Squirrel, where it lies to the left of the epiploön, as in the Marmot: it follows, similarly suspended, the great curve of the stomach in the Mole-rat (Bathyergus), being thickest at the left and upper end: in the common Rat the spleen has an oblong triangular form: in the Vole it is broader at the lower than at the upper end: in Capromys it has an elongate trihedral form, broadest at the lower end: in Lagostomus the spleen is triangular, with the upper and anterior angle most produced.' It varies from the round to the oblong shape in the Porcupines and Agoutis, and occasionally a small detached spleen is added, in the epiploic suspending duplicature. Hunter notes, in the Capybara, the close resemblance of the spleen in shape to that of Man;2 and it is less elongate than usual in the Guinea-pig. In Leporida it resumes its narrow elongated figure. In Dasypus Peba the spleen is elongate and three-sided; I found it 24 inches in length; in contact with the pancreas: in Das. 6-cinctus the spleen is broader and flatter, and there was a small supplemental spleen in my subject.3 In the three-toed Sloth the spleen is an inch in length, oblong, thickest at the lower end, suspended in the epiploön: in the two-toed Sloth it is almost round, flat, and thin, and closely attached to the second compartment of the stomach,

1 ccxn". p. 176.

2

CCXXXVI. vol. ii. p. 213.

3 cxxvi". pp. 143, 157.

but in contact with the pancreas. In some Cetacea the spleen is remarkable for its subdivisions; the largest in the Porpoise, fig. 354, equals a walnut, h; the others, to the number of four, five, or six, ib. i, i, are of much smaller size: in the Whales (Balænoptera) the spleen is, usually, single, but smaller relatively. In Sirenia it appears to be always undivided; presenting an oval form in the Dugong, and measuring 4 inches in length and 14 inch in breadth.

In the Elephant the spleen is long and flat; it measured in a half-grown Indian kind 3 feet 10 inches, its extreme breadth being 8 inches.

2

In the Hyrax the spleen is broad, flattened, semilunar, with occasionally a narrow process from its middle, like a handle: its length is 2 inches, its breadth 1 inch. In the Rhinoceros the spleen is elongate, subtrihedral: in my male subject it measured in length 3 feet 6 inches, and 1 foot 4 inches in greatest breadth.' The spleen is elongate, and flattened in the Horse, broadest at the upper end. In the Wart-hog (Phacochorus) the spleen is a long flattened ellipsoid body, 11 inches in length, and 2 inches across its broadest part, which is at the middle. The spleen has a similar form in the Baby roussa (Sus Babyrussa): in the common Hog it is elongate and trihedral. The spleen is elongate and flattened in all Ruminants; the inner edge is sometimes attached to the crura of the diaphragm: it is broader, at one end, in the Cow, Reindeer, and Giraffe, than in other Ruminants. In one Giraffe the spleen was 10 inches long, and 7 inches broad: in another of the same stature it was 9 inches long, and 5 inches broad: in both of an oval form, and not more than 1 inches at the thickest part.3

If a spleen be injected with alcohol and hardened therein previous to section, the intertrabecular spaces are seen to be larger in Ungulata than in Carnivora. In the Horse such spaces are then seen to intercommunicate by circular apertures.

In a Seal (Phoca vitulina) I found the spleen a flattened body with an irregular notched margin, measured 5 inches in length. It was attached to the epiploön in such a manner that it could be drawn away for some distance from the stomach, and in the intervening membrane were situated a number of small dark glandular bodies from the size of a horse-bean to that of a pea, resembling the omental splenules in the Porpoise: these were not found in a second specimen.

In a setter-dog the spleen was oblong, 10 inches long by 3 inches

1 v". p. 44.

2 ccx". p. 68.

3 XCVII". p. 228.

VOL. III.

at the broadest part: its serous coat, as in most quadrupeds, is derived from both layers of the epiploön, which are reflected from one margin to the stomach, and by the opposite to the dorsal abdominal walls. In the Felines the spleen commonly presents an elongate trihedral form, attached to the stomach by the duplicature extended from the angle formed by the meeting of the two lesser sides.

In the Aye-aye the spleen is an elongate, trihedral body, bent at nearly a right angle upon itself, the lower portion being nearly half the length of the upper one; it is suspended in the epiploon at the left and lower curve of the stomach. The spleen presents a like shape and position in the Lemurs: but is less bent in Lemur Mongoz. The spleen is elongate and straight in Platyrhine Monkeys; it becomes broader and thicker in Catarhines: it shows a subtriangular form in the Baboon (Papio porcarius), where one angle is attached to the stomach, another to the kidney, and a third projects freely into the epiploön: in tailless Apes the spleen more resembles in shape, attachments, and in the source of its serous investment, that of Man.

The loose nature of the suspension of the spleen somewhat affects the value of the remarks on its various positions in Mammalia, given in XII. tom. iv. pl. ii. p. 617, where it is said to be near the pyloric end of the stomach in a Nyctinome and a Noctilio, a Phyllostome and a Megaderm among Bats, while in other species of these genera it was found nearer the cardia: in a Vespertilio and Rhinolophus it was observed to be bent round the great curvature of the stomach. In Pteropus and Galeopithecus the spleen retains its common position applied to the cardia: it is relatively smaller in frugivorous than in insectivorous and sanguivorous Bats, but is generally long and narrow: it is triangular in Galeopithecus. In Insectivora the spleen is loosely suspended in the epiploön from the cardiac cul-de-sac; it is relatively largest in the more carnivorous of the order, e.g., the Tenrecs.

The spleen is larger in the omnivorous and quasi-carnivorous Rodents, e.g., the Rats, than in the vegetarian majority of the order: it is relatively larger in Carnivora than in Ungulata. The amount of hydro-carbonates to be eliminated by the liver would seem to influence the capacity of the alterative receptacle of the great proportion of the blood which is supplied to the bile-making With reference to the hypothesis of sanguifaction it may be remarked that in no Mammalian order is the mass of blood so great, or so full of blood-discs, as in the Cetacea; yet in them the spleen has its least relative size.

organ.

§ 351. Thyroid of Mammalia.-The representative beginnings of the vasoganglion commonly known as the thyroid gland' are noticed in vol. i. p. 564 (Fishes, Reptiles), and in vol. ii. p. 230 (Birds): but this organ is recognisable, without ambiguity, only in the present class. Here it is locally related to the windpipe, and has received its name from its proximity to the shield-shaped cartilage of the larynx in the human subject. It consists of a pair of oblong, rounded masses; in some, especially higher gyrencephalous Mammals, united as in Man by a transverse band of like substance crossing the sternal aspect of the air-tube.

The proper tunic of the thyroid is a thin layer of condensed areolar tissue, from the inner surface of which proceed septal or trabecular processes, partitioning its substance into lobules, and ultimately into minute bags of vesicles. The analogy to the structure of the spleen is close, but the frame-work is much less dense and fibrous and the vesicular structure, instead of receiving the blood directly, is filled with a solution of fibro-albuminoid, proteine, or myeline substance derived therefrom. The quantity of blood sent to the thyroid is much more than would be needed for mere nutrition: it is derived from arteries, not constantly rising just beyond the points where the arteries to the brain are given off from the large trunks, but varying according as the length of the neck in Mammals may affect the relative position of the thyroid to those trunks: thus in the Giraffe and most Ungulates the arteries supplying the thyroids come off from the contiguous part of the carotids. There may be two or three branches from the common carotid (Lutra); and the distinction between lower thyroid arteries' from the subclavian, and upper thyroid arteries' from the ectocarotid, hardly begins to be established before the Quadrumanous order is reached. The ultimate ramifications of these arteries form close-meshed plexuses upon the limitary membrane or capsule of the vesicles; such capillaries present a diameter of fromth to 30th of an inch. The blood is returned by veins joining in most Mammals the external jugular; and in Quadrumana and Man the internal jugular: but with varieties in this respect.

3000

6

The effect of formifaction,' or assumption of shape and definable size, by the colloid,'' proteine,' or 'myeline' elements of the solution filling the thyroid vesicles, is shown in the sections of such from the Hedgehog, fig. 435, and the Bullock, fig. 437, and in portions of such lining, or adherent formed matter, from the thyroid vesicles of a Rabbit, fig. 436. In these instances

the forms have been described as an epithelial stratum, consisting usually of nuclei set closely together in a scanty basis substance, fig. 435, which is either feebly granular or of a somewhat oily aspect:' their nucleoli are not always visible, and vary in number from one to four or five. The nuclei are always vesicular, bounded by a strongly marked envelope, and have a mean diameter of 50th inch.'1 But the formed lining

00

substance often presents, as in figs. 435 and 437, the condition of delicate vesicles, without nucleus, with contents mostly pellucid, sometimes faintly granular. Dr. Jones observes :-'I am inclined to believe that they originate in the nuclei, which undergo a kind of expansion, at the same time losing their nucleoli.' Emancipating himself for a moment from the 'generative' theory in reference to the progress of the nucleus from its primitive condition to a further stage of cell-development,' he candidly admits it to be worth remarking that it' (the stage) may be artificially produced by adding to the specimen some coagulating reagent, which speedily solidifies a film of albuminous plasma around the nuclei, and thus produces very good imitations of cells.'2

Analyses of the contents of the thyroid have shown or rendered it very probable that they are albuminoid, yet not in the state of ordinary fluid albumen, and that gelatine is sometimes an ingredient: among the salts are chloride of sodium and a trace of alkaline sulphate: crystals of triple phosphate and of oxalate of lime occur in the cavities.3

CCLXXIX. p. 1104.

2 Ib. p. 1105. For the conditions and degree in which this and most other phenonomena of so-called 'cell-development' may be artificially manifested, see ccx" and ccx", especially the latter important contribution to the philosophy of physiology. 3 CCLXIX. p. 1106.