vessel (Figs. 26, b, b; 46, m, m), and which look like round cells or nuclei scattered through the tissue. These are fibre-cells seen in apparent transverse section. The layer formed by the middle coat may be most distinctly seen, however, after the addition of acetic acid, which causes the appearance of a great number of oblong nuclei.

It is this layer which, generally speaking, confers upon the arteries their specific character, and distinguishes them most clearly from the veins. There are, indeed, numerous veins in the body which possess considerable layers of muscular tissue, for example, the superficial cutaneous veins; still, in the case of the smaller vessels, the occurrence of a distinctly marked circular-fibre coat is so peculiarly characteristic

of arterial vessels, that, wherever we meet with such a structure, we are at once inclined to assume the vessel to be arterial.

Fig. 46. A minute artery from the sheath of the tendon of one of the extensors of a hand just amputated. a, a. External coat. m, m. Middle coat, with welldeveloped muscular layer. i, i. Internal coat, partly with longitudinal folds, partly with longitudinal nuclei, in the side-branch brought well into view in consequence of the two external coats having been torn away. 300 diameters.

These vessels, which must be included among the larger ones, although even when full of blood they only appear to the naked eye like red filaments, pass gradually into smaller ones, and with a power magnifying three hundred diameters we see them breaking up into branches, into which, even when they are very small, the three coats are at first continued. It is only in the smallest branches that the muscular coat finally disappears, the intervals between the individual transverse fibres becoming wider and wider, and the internal coat (the nuclei of which lie in a longitudinal direction and cross those of the middle coat at right angles (Fig. 26, D, E)), at the same time appearing more and more distinctly through it. The external coat also may be followed for a short distance farther (being in many places, as in the brain, rendered more evident by the interspersion of pigment or fat, Fig. 26, D, E), till at last it also becomes lost to view, and only a simple capillary remains (Fig. 3, c). The general supposition, therefore, is that the proper capillary membrane most nearly corresponds to the internal coat of the larger vessels, and it is usually considered that the more complete a vessel becomes, the greater is the number of the coats which develop themselves around it. The real developmental relations which these parts bear to one another have, however, been by no means accurately determined.

Within the true capillary region there is nothing further worthy of notice in the vessels than the nuclei I have previously mentioned, which correspond to the longitudinal axis of the vessel, and are so imbedded in its membrane, that it is impossible to discover any traces of a surrounding cell-wall The capillary membrane is seen to be quite uniform, absolutely homogeneous and continuous (Fig. 3, c). Whilst even as lately as twenty years ago it was a matter of discussion whether there did not exist vessels which were destitute of true walls, and were nothing more than channellings or excavations in the parenchyma of organs, as well as whether vessels

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could not be produced by the formation of new tracks in communication with the old channels by the forcing asunder of the neighbouring parenchyma; there can, at the present time, be no longer any doubt as to the vascular system's being everywhere continuously closed by membranes. In these it is not possible to descry any porosity. Even the minute pores, which have recently been observed in different parts, have not, up to the present time, met with their counterparts in the capillary membrane, and when the porosity of this membrane is spoken of, the expression can only be admitted in a physical sense, as applying to invisible, really molecular interstices. A film of collodion is not more homogeneous, nor more continuous, than the membrane of a capillary. A series of possibilities, which used to be admitted, as that, for example, the continuity of the capillary membrane did not exist at certain points, simply fall to the ground. A "transudation or diapedesis of the blood through the walls of vessels. without the occurrence of any rupture cannot for an instant be admitted; and although we cannot in every individual case point out the exact site of the rupture, it is, notwithstanding, quite inconceivable that the blood with its corpuscles should be able to pass through the walls in any other way than through a hole in them. This is such a very natural deduction from ascertained histological facts, that all discussion upon the point is impossible.

After the capillaries have pursued their course for a time, small veins begin gradually to form out of them, and generally run back in the neighbourhood of the arteries (Fig. 45, v). In them the characteristic circular-fibre coat of the arteries is in general wanting, or at least it is very much less developed. In its place we find in the middle coat of the larger veins toughish layers, which are not characterised so much by the absence of muscular elements as by the greater abundance of elastic elements which run in a longitudinal direction and are found in greater or less

quantity in different localities. In an inward direction there follow next the softer and more delicate layers of connective tissue of the internal coat, and lying on this is found, in the last place, a flat, extremely translucent layer

of epithelium, which is very prone to protrude out of the cut end of the vessel, and gives the impression of spindle

Fig. 47. 4. Epithelium from the femoral artery (Archiv f. path. Anat.,' vol. iii, figs. 9 and 12, p. 596). a. Division of nucleus. B. Epithelium from veins of considerable size. a, a. Largish, granular, round, uni-nuclear cells (colourless blood-corpuscles?). b, b. Oblong and spindle-shaped cells, with divided nuclei and nucleoli. c. Large, flat cells, with two nuclei, of which each has three nucleoli, and is in process of division. d. Coherent epithelium, with the nuclei in a state of progressive division, one cell having six nuclei. 320 diameters.

Fig. 48. Epithelium from the vessels of the kidney. 4. Flat, spindleshaped cells with longitudinal folds and large nuclei from a new-born child. B. Ribbon-like, nearly homogeneous, plate of epithelium with longitudinal nuclei from an adult. 350 diameters.

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shaped cells, so that it may easily be mistaken for spindleshaped muscular cells. The smallest veins likewise possess this epithelium, but, with this exception, are, properly speaking, entirely composed of connective tissue provided with longitudinal nuclei (Fig. 45, v).

These relations undergo no essential change even when the individual constituents of the vascular system experience the most extreme enlargement. This is best seen in pregnancy, in which not merely in the uterus, but also in the vagina, the Fallopian tubes, the ovaries, and the ligaments of the uterus, both the large and small arteries and veins as well as the capillaries exhibit a very high degree of dilatation, so that the rest of the tissue, in spite of its having likewise in no inconsiderable degree become enlarged, is thereby virtually thrust into the back ground. Nevertheless, however, parts of this puerperal sexual apparatus are extremely well adapted for displaying the relation between the histological elements and the vascular (arterial) districts. In the fimbriae of the Fallopian tubes, for example, every plexus or loop formed towards the borders by the greatly dilated capillaries encloses a certain number of large connectivetissue cells, of which only a few lie in immediate contact with the vessels. In the alæ vespertilionum we find, moreover, very beautifully displayed, a condition which is of frequent occurrence in the appendages of the generative organs, and similar to what we lately considered in the scrotum; the vessels, namely, are accompanied by flat bundles of smooth muscle in considerable quantity which do not belong to them, but only follow the course of the vessels, and in part receive the vessels into them. This is an extremely important feature, inasmuch as the contraction of these ligaments, in which muscular tissue is not generally considered to exist, is by no means solely to be ascribed to the blood-vessels, as James Traer only a short time ago endeavoured to establish; on the contrary, we