these reservoirs of blood at the depths to which they retreat when harpooned, explain the profuse and lethal hæmorrhage which follows a wound that, in other Mammalia, would not be fatal.

6

The

In the Ungulates a left azygos co-exists with a single (right) precaval. In the Hog the azygos trunk passes forward, left of the aorta, crossing that vessel below the arch and curving over the left auricle, and enters the right. Hunter notices its attachment to the left auricle and its analogy, in both the Hog and Fallowdeer, to the left precaval in Birds. In the Corinne Antelope (A. Dorcas) Hunter observed two azygos veins, the left being the larger';' a right azygos exists in the Ox, and is larger in the Horse, receiving blood from several of the left intercostal spaces. oblique vein at the back of the left auricle is large in the Dromedary and Tapir, and represents the remnant of a left azygos. The portal vein shows valves in some Ruminants. In the Rhinoceros the right precaval receives the right or common azygos close to its termination at the upper part of the right auricle two inches above this it receives the right vertebral vein which is about half an inch in diameter; two inches above this it is formed by the junction of the left brachiocephalic. At the concavity of the great vein formed by this junction, the bronchial veins and some small pericardial veins enter. The upper part of the precaval receives the two large jugular veins close together, so that a proper vena innominata' can scarcely be said to be formed. The left vena azygos, which is formed by the union of a few intercostal veins of the same side, terminates in the left subclavian vein, which receives separately the left vertebral vein from the neck. The right or principal azygos receives the intercostal veins of both sides as far forwards as its entry into the precaval vein: the Rhinoceros in this structure agrees with the Horse. The coronary vein receives a small pericardial vein, which descends along the back of the left auricle, before it terminates with the inferior cava, at the base of the right auricle.

With the relatively long and narrow thorax of the hoofed and most other Mammals the pre- and post-caval trunks are correspondingly longer than in Man; the length of the thoracic part of the post-caval being as is the distance of the right auricle from the diaphragm, and also from the dorsal region: the base of the heart being further from the back as well as from the midriff than in Man.

In Carnivora, Quadrumana, and Bimana the blood from the 2 Ib. p. 147.

1 CCXXXVI. vol. ii. pp. 124, 140.

428

head and pectoral limbs, and intercostal spaces is poured by a single precaval into the auricle. The lower intercostal veins of the left side unite to form a hemi-azygos which joins the right or main azygos vein. The plexiform disposition of the veins continues in those surrounding the myelon up to and including Man. The diploic plexuses are the networks of veins which exist in and among the cancellated tissue of the bones. In those of the cranium they form large irregular meshes of ampullated veins. These vessels are very unequal in size, are subject to dilatations, and frequently end in culs-de-sac. In the looser texture of the vertebral centrums a vertical section, as in fig. 428, exposes the anterior, a, and posterior, b, portions of the myelonal plexus; the transverse channel of anastomosis, c, with the sinus centri,' d, which bifurcates to unite with the vena superficialis centri,' e. Many veins within the cranium are included in spaces formed by the separation of the lamina of the dura mater, and do not admit of being dilated beyond a certain size: these 'sinuses' empty themselves, in Marsupials and Ruminants and some other quadrupeds, into temporal veins, as well as into the internal jugulars; but in Carnivora, Quadrumana, and

lumbar vertebra.

Man, almost wholly into the internal jugular vein.

6

In all Mammals may be found the 'superior longitudinal sinus,' fig. 429, s, uniting at t, the 'torcular Herophili' with the lateral sinuses, e. Besides these are the smaller petrosal sinuses,' superior and inferior, and the cavernous sinuses' which are reservoirs of venous blood on each side of the sella turcica, crossed by interlacing sclerous fibres. The cavernous sinuses communicate with each other by the circular sinus,' and also with the petrosal ones by the transverse sinus.' All the sinuses are devoid of valves, and, by their freedom of intercommunication,

and hindrance to dilatation, prevent any local congestion of blood pressing upon the brain.

$350. Spleen of Mammalia.-As with the absorbents, so with the blood vessels, some change

their tubular for a lacunar or cellular form in certain parts, where the blood undergoes, or receives elements of, change; such parts, resembling' glands,' are so called, with the qualifying epithets of vascular' or 'ductless. Of these 'vasoganglions' the chief is the spleen.

In Mammals this organ is relatively larger than in lower Vertebrates: it is mainly appended to the artery that supplies the pancreas, with the left end of which gland it is in close connection, and consequently lies to the left or behind the stomach, to which it is

This

429

430

a

attached by the fold of perito- Venous sinuses of dura mater, from behind, Human. neum noted at p. 500. membrane covers the whole spleen, except the hilus,' where its two folds support the splenic vessels and form the 'gastro-splenic' ligament. The serous tunic is less. intimately adherent to the fibrous or proper capsule in most lower Mammals (Ruminants, e.g.) than in Man; where the separation only takes place at the hilus. The proper coat consists mainly, and in Man wholly, of white and yellow fibres, the former arranged in bands, the latter in an irregular network. With these are blended, in some lower Mammals (Dog, Pig, e.g.), filamentary fusiform bodies with a nucleus, fig. 430, a, called fibre-cells of unstriped muscle' in CCVIII". The fibrous tunic is reflected into the interior at the hilus, in the form of sheaths accompanying the vessels, most complete in the human spleen: from the exterior of which sheaths, and more abundantly from the inner surface of the proper capsule, are sent off white elastic trabecular' bands, which form a reticular bed for

6

Nucleate filament or muscle-cell,' from tunica propria of the spleen of a dog: magnified 350 diam. CCVIII".

the proper tissue, throughout the whole organ. This tissue consists of the lienine' or spleen-substance and of splenic corpuscles.' If a portion of the trabecular tissue be treated with acetic acid, muscular fibre-cells may be seen, as at a', with their nuclei b, intermixed with the yellow elastic fibres e, fig. 431. The demonstration is easiest in

431

432

Trabecula from the spleen of a Pig: magn. 350 diam. CCVIII".

433

Splenic or 'Malpighian' corpuscles, on branches
of an arteriole: from the spleen of the Pig.
magn. 10 diam. ccvIII".

the most delicate plates of the trabecular tissue, especially in quadrupeds. The splenic corpuscles, fig. 432, c, c, are whitish spherical bodies imbedded in the 'lienine;' most constant and conspicuous in ruminant, equine, and some other quadrupeds; less conspicuous, or wanting, in adult human spleens, especially after lethal disease. They are elliptical, averaging one-sixth of a line in diameter, and are attached by short peduncles to splenic arterioles a, b, the peduncles being continuous with the sheaths accompanying those vessels. Treated with a little dilute alkali, the proper wall of the corpuscle, fig. 417, a, is rendered more distinct, and the elastic fibres of the same, b, may be seen, in connection with the branch of the arteriole c, to which it is appended. The corpuscular capsule is filled by a semi-fluid greyish mass, including nucleated corpuscles, fig. 433. They have suggested

Elementary forms of

substance in splenic

corpuscle, Ox, magn.

350 diam. CCVIII".

434

many hypotheses, their comparison, by Kölliker, to the cells in the sinuses of lymphatic ganglions, appearing to be most acceptable. The 'lienine' is a soft mass, in colour passing from reddishbrown to bright red on exposure to air, filling up all the interstices between the larger partitions and vessels. It consists of fine blood vessels, lienine cells, delicate fibres or bands, and blood in various states. The lienine cells, fig. 434, vary in size from 3th to 15th of a line, are pale in colour, with a dark nucleus with them are cells with smaller corpuscles, caudate corpuscles, and free nuclei; all exemplifying the size-limiting or shape-inducing property of the viscid materials, proteine and myeline, under the reaction of albumino-serous solution: fallaciously suggesting the continuous process of cell-growth by which new cells are formed around nuclei, and old ones disappear;" as also the development of blood-discs within cells.' and 434 merely exemplify some among the manifold forms under which colloid elements aggregate in definite spaces, under such influences as the spongy reservoir of the spleen affords.

72

с

Elementary forms of substance in lienine, Ox; mag. 350 diam. cevi".

The figures 433

The splenic artery, especially when the pancreatica magna and other branches to the pancreas are not called upon to supply materials for the energetic and fitful action of that gland, must pour more blood into the splenic reservoir than is needed for the mere nutrition of the organ, and consequently the blood must there undergo change. But the spleen receives too small a proportion of the circulating mass to have any definite influence on the manufacture or general condition of blood. Such changes as are effected in the splenic locality more probably relate to the functions of the gland to which the altered blood is exclusively carried and it is to be noted that the splenic vein is the largest of the constituent channels of the portal one. The most significant fact in the Comparative Anatomy of the spleen is its correlative development with the pancreas and its reception of blood from the termination of the artery mainly supplying the pancreas in its course to the spleen.

1 cсvi". p. 781.

2 Ib. p. 782, figs. 531, 532. These, and the figures 529, 530, represent nothing specifically distinct from the results of formifaction under similar conditions in other localities both in and out of the living body.

The supply of the colouring matter of the bile from hæmatin set free in the spleen has been suggested. Extirpation of the spleen chiefly affects the biliary secretion. The condition of the spleen in Hematocrya negatives its being the seat of the manufacture of blood-corpuscles.