and other allied textures. In infants and young persons, the tissues are comparatively more vascular than in after-life.

The figure of the capillary network is not the same in all textures. In many cases the shape of the meshes seems accommodated to the arrangement of the elements of the tissue in which they lie. Thus in muscle, nerve, and tendon, the meshes are long and comparatively narrow, and run conformably with the fibres and fasciculi of these textures. In other parts, as in the lungs, in fat, and in secreting glands, the meshes are rounded or polygonal, with no one dimension greatly predominating. In the papillæ of the skin and mucous membranes, the vessels of the network are often drawn out into prominent simple or ramified loops. The smallest arteries and veins pass by gradual transition into the capillary vessels, and their finest offsets approach very near to these in structure; these may therefore be conveniently considered along with the capillary vessels.

Structure of the capillaries.-The wall of the capillaries proper is formed entirely of a simple epithelial layer, composed of flattened lanceolate cells joined. edge to edge, and continuous with the corresponding layer which lines the arteries

Fig. 428.-CAPILLARY VESSELS FROM THE BLADDER OF THE CAT,

MAGNIFIED (after Chrzonszczewsky).

The outlines of the cells are stained by nitrate of silver.

and veins. The outlines of the cells or their lines of unction one with another may be made apparent by .trate of silver (fig. 428); while the nuclei, which show a well-marked network of karyoplasm, may be brought into view by logwood or carmine. Commonly there are not more than two or three such cells in the cross section of a capillary. At the points of junction of the capillaries the cells are usually broader and not spindle-shaped, but radiate, with three or four pointed branches fitting in between the cells of the three or four adjoining vessels which meet at the spot (fig. 428, c, c, c').

In capillaries which have been submitted to the action of nitrate of silver, there is here and there to be seen between the cells of the capillary wall an increase in amount of the intercellular substance, appearing as an enlargement of the fine line of the silver deposit. To these gaps in the capillary wall, which however are closed by intercellular substance, J. Arnold has applied the term "stigmata;" they are analogous to the "pseudo-stomata " found between the epithelium-cells of a serous membrane. It is probable that the white blood corpuscles, when migrating from the blood-vessels, pass between the epithelium-cells, especially in the situation of the stigmata.

Branched cells of the surrounding areolar tissue are found connected intimately with the cells forming the capillary wall. This connection occurs almost everywhere, but it is more obvious in parts which are pervaded by a supporting network of retiform connective tissue, such as the substance of the lymphatic glands, the solitary and agminated intestinal glands and adjacent mucous membrane, where the small vessels and capillaries may even obtain a continuous covering from the reticulating processes of the cells. This coating has been named by His, adventitia capillaris.

Outgrowths from the capillary wall have been described by Stricker as occurring not only in the progress of development, in the manner to be afterwards detailed, but also in the fully developed capillaries of the frog; and contraction both of the whole capillary wall and also of the individual cells of young capillary vessels has been described (Stricker, Tarchanoff), but it is not known whether the walls of the capillaries of the adult mammal possess any appreciable contractility.

Structure of the small arteries and veins. In vessels a little larger than the capillaries, there is added outside the epithelial layer, a layer of plain

muscular tissue, in form of the usual long contractile fibre-cells, which are directed across the length of the vessel. The elongated nuclei of these cells may be brought into view by means of acetic acid or by staining fluids (fig. 430). This layer corresponds with the middle coat of the larger vessels. In the smallest vessels in which it appears the muscular cells are few and apart, and a single long cell may turn spirally round the tube (Lister); in larger vessels, especially those of

the arterial system, the muscular cells are more closely arranged. Outside the mus

cular coat is the areolar or connective tissue coat, containing fibres and connective tissue corpuscles, with longitudinally placed nuclei.

In vessels of of an inch in diameter, or even less, the elastic layers of the inner coat may be discovered (fig. 430, A, 8), in the form generally of homogeneous or fenestrated membrane, more rarely of longitudinal reticulating elastic fibres. The small veins differ from arteries of corresponding size, chiefly in the inferior development of

their muscular tissue; the lining cells of the arteries also are very much longer and narrower than those of the veins. These differences, as well as the comparative size of corresponding vessels, are well shown in the accompanying figures (429 and 430).

The only open communication between the arteries and the veins, is by means of capillary vessels as above described, unless in the maternal part of the placenta and in the interior of erectile organs, in which small arteries may open directly into wide venous cavities without the intervention of capillaries. Moreover, in the spleen the arterial capillaries do not at once pass into the commencements of the veins, but open into the interstices of the organ, from which the minute veins collect the blood.

But it would appear that in certain parts small arteries may pass into small veins without the intervention of true capillaries (Sucquet, Hoyer).

Arterial glands.-At the upper end of the common carotid (carotid gland) and in front of the apex of the coccyx (coccygeal gland, Luschka), are found small solid-looking bodies of a

somewhat glandular appearance, but composed almost entirely of a plexus of minute arteries, which are derived in the one case directly from the carotid, in the other from the middle sacral. The plexiform vessels are invested by one or more layers of granular polygonal cells, apparently like those found in the interstitial tissue of some other organs (testis, ovary,

Fig. 430.-A SMALL ARTERY A, WITH A CORRESPONDING VEIN B, TREATED WITH ACETIC ACID, AND MAGNIFIED 350 DIAMETERS (after Kölliker).

a, external coat with elongated nuclei; B, nuclei of the transverse muscular tissue of the middle coat (when seen endwise, as at the sides of the vessel, their outline is circular); 7, nuclei of the epithelium-cells; 8, elastic layers of the inner coat.

thyroid, suprarenal bodies). The whole is invested by connective tissue, which also penetrates between the vessels of the so-called gland, and itself contains numerous granular cells. The true nature and function of these peculiar structures is entirely unknown, but they are probably of embryological significance.

The first vessels which appear in the ovum are formed in the mesoblast, and the process subsequently goes on in the same layer and in its derivatives in all parts of the animal body. New vessels, also, are formed in the healing of wounds, in the restoration of lost parts, and in the production of adventitious growths. The process is in every case essentially the same.

The first vessels of the embryo, both of the chick and mammal, are formed in the vascular area, and originate from some of the cells of the mesoblast in that situation (fig. 431). Vacuoles are formed within the cells, and as they increase in size run together, and a cavity filled with fluid is in this way produced in the interior of the cell. The nucleus of the cell has meanwhile become multiplied, while blood-corpuscles are formed within the cell-cavity in the manner already described in connection with the blood (p. 217). The cells, whilst these changes are going on, increase largely in size, especially in the chick, where they form vesicles (fig. 432), visible to the naked eye as minute reddish specks, which have been known since the time of Pander as "blood-islands." The cells are united to one another by their processes, and after a time the cavities become extended into the cell-processes, so that a network of vessels is by this means produced.

The wall of these primary vessels is therefore composed at first merely of the protoplasm of the original embryonic cells with nuclei, derived by division from the

original nuclei of those cells, imbedded in it here and there. Subsequently the protoplasm becomes differentiated around the nuclei into the flattened cells which

d

a

bl.

Fig. 431.-PART OF THE NETWORK OF DEVELOPING BLOOD-VESSELS IN THE VASCULAR AREA OF THE GUINEA-PIG. (E. A. S.)

bl, blood-corpuscles becoming free in an enlarged and hollowed-out part of the network. The smaller figure on the left represents a of the larger figure, more highly magnified, showing the vacuolation of the cell; d, a nucleus within it undergoing division.

compose the wall of the capillaries, and which form the lining membrane of the arteries and veins. The remaining coats of the larger vessels are developed later,

Fig. 432.-CELLS FROM MIDDLE LAYER OF CHICK'S

BLASTODERM UNDERGOING DEVELOPMENT INTO

BLOOD-VESSELS. MAGNIFIED. (Klein.)

a, cavity of cell; b, wall of cell; f, f, cells not yet hollowed out; d, blood-corpuscles.

from other cells which apply themselves to the exterior of the previously simple endothelial tubes and produce the plain muscular and other tissues of which those coats consist.

Within the body of the embryo, vessels are formed in like manner from cells within the connective tissue, especially in rapidly growing vascular organs like

the liver. One of the most favourable objects for the study of the development of the blood-vessels and their contained blood-corpuscles is afforded by the subcutaneous tissue of the new-born rat, especially those parts in which fat is being deposited.

Here we may observe that many of the connective tissue corpuscles are much vacuolated, and that the protoplasm of some of them has a decided reddish tinge (fig. 433, h). In others the red matter has become condensed in the form of globules within the cells (h', h", &c.), varying in size from minute specks to spheroids of the diameter of a blood-corpuscle, or more. At some parts the tissue is completely studded with these cells, each containing a number of such spheroids, and forming, as it were, "nests" of blood-corpuscles or minute "blood-islands." The cells become elongated and pointed at their ends, sending out processes also to unite with neighbouring cells. At the same time the vacuoles in their interior become enlarged, and coalesce to form a cavity within the cell (fig. 434, a), in which the

reddish globules, which are now becoming disk-shaped (b), are found. Finally the cavity extends through the cell-processes into those of neighbouring cells and into those sent out from pre-existing capillaries (fig. 434, c), but a more or less extensive

Fig. 433.-COMMENCING DEVELOPMENT OF ANGIOBLASTIC CONNECTIVE TISSUE CELLS INTO BLOOD-VESSELS. FROM THE SUBCUTANEOUS TISSUE OF THE NEW-BORN RAT. (E. A. S.)

h, a cell containing hæmoglobin in a diffused form in the protoplasm; h', one containing coloured globules of varying size, and vacuoles; h", a cell filled with coloured globules of nearly uniform size; f, f', developing fat cells.

Fig. 434.-FURTHER DEVELOPMENT OF CONNECTIVE TISSUE CELLS INTO CAPILLARY BLOOD-VESSELS.

(E. A. S.)

a, an elongated cell with a cavity in its protoplasm occupied by fluid and by blood-corpuscles which are still globular; b, a hollow cell the nucleus of which has multiplied. The new nuclei are arranged around the wall of the cavity, the corpuscles in which have now become discoid; c, shows the mode of union of a hæmapoietic" cell, which in this instance contains only one corpuscle, with the prolongation (b) of a previously existing vessel. a, and c, from the new-born rat; b, from the foetal sheep.

Fig. 435.-ISOLATED CAPILLARY NETWORK FORMED BY THE JUNCTION OF SEVERAL HOLLOWED-OUT CELLS, AND CONTAINING COLOURED BLOOD-CORPUSCLES IN A CLEAR FLUID.

c, a hollow cell the cavity of which does not yet communicate with the network; p, p, pointed cellprocesses, extending in different directions for union with neighbouring capillaries.

capillary network is often formed long before the connection with the rest of the vascular system is established (fig. 435). Young capillaries do not exhibit the wellknown lines when treated with nitrate of silver, for the differentiation of the hollowed