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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

Fig. 47.

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

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Fig. 47. A. Epithelium from the femoral artery (' Arcbiv 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, e. 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. A. 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.

VESSELS IN PREGNANCY. 115

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 alae 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 find thickish, flat bundles of muscle which run through the middle of the ligaments, and during menstrual excitement enable contractions to take place, similar to those which we can follow with such great distinctness in external portions of the genital passages.

If now the question be raised how far the individual elements of the vessels are of importance in the body, it is at once evident that the contractile elements play the most important part in the coarser processes of the circulation, whilst the elastic constituents come next, and the simply permeable, homogeneous membranes last. Let us first consider the import of the muscular elements, and more particularly in those vessels which are chiefly provided with them, namely the arteries.

When an artery is acted upon by any influence which causes a contraction of its muscular tissue, it must of course become narrower, inasmuch as the contractile cells lie in rings around the vessel; this contraction may under certain circumstances proceed until the canal is almost entirely obliterated, and the natural consequence then is that less blood penetrates into the corresponding part of the body. When therefore an artery is in any way exposed to a pathological irritant, or when it is excited by some physiological stimulus, its proper action cannot be displayed in any other way than by its becoming narrower. Now, indeed, that the muscular elements of the walls of the vessels have become known, the old doctrine might again be taken up, that, namely, the vessels, like the heart, originated a kind of rhythmical pulsating movement, which was capable of directly furthering the onward movement of the blood, so that an arterial hyperaemia would be the result of an increased pulsation in the vessels.

We are indeed acquainted with one isolated fact which is a proof that a real rhythmical movement does take place in CONTRACTION OF ARTERIES. 117

the arterial walls; and this was first observed by Sehiff in the ears of rabbits. Its rhythm, however, does not at all correspond with that of the well-known arterial pulsation; the only counterpart to it exists in the movements which had previously been observed by Wharton Jones in the veins of the wings of bats, and proceed in an extremely slow and quiet manner. I have studied these phenomena in bats, and convinced myself that the rhythm coincides neither with the cardiac nor the respiratory movements; it is quite a peculiar, but comparatively not very forcible, movement, and takes place after tolerably long pauses, longer ones than are observed in the case of the circulation and shorter than those which occur in respiration. In the ears of rabbits also the contractions of the arteries are far slower than the cardiac and respiratory movements.

After excluding these phenomena, which manifestly ought not to be explained in such a way as to support the old view of the local occurrence of pulsation, the essential fact remains, that the muscular fibres of a vessel contract upon the application of every stimulus which sets them in action, but that this contraction is not propagated in a peristaltic manner, but is confined to the spot irritated, or at most extends a little beyond, and continues for a certain length of time at this spot. The more muscular the vessel is, the more lasting and forcible is the contraction and the greater is the obstruction experienced by the current of blood. The smaller the vessels, the more rapidly, on the contrary, do we see the contraction succeeded by a dilatation, which, however, is not in its turn followed by a contraction, as it would have to be to constitute a pulsation, but persists for a longer or shorter time. This dilatation is not of an active, but of a passive, nature, and results from the pressure of the blood upon the wall of the vessel which has become fatigued and opposes less resistance.

If we now proceed to examine the phenomena which are usually grouped together under the title of active hyperaemia, there can be no doubt but that the muscular tissue of the arteries is generally essentially concerned therein. We very commonly find we have to deal with processes in which the muscular fibres of the vessels have really been stimulated, and the contraction is succeeded by a state of relaxation,

Fig. 49.

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Fig. 49. Irregular contraction of small vessels from the web of a frog's foot after the application of stimuli. Copied from Wharton Jones.

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