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MARCH 3, 1858.
NUTRITION AND CIRCULATION.
Arteries. — Cnpillaries. — Continuity of their membrane.—Their porosity.— Haemorrhage by transudation (per diapedesin).—Veins.—Vessels during pregnancy.
Properties of the walls of vessels:
1. Contractility.—Rhythmical movement.—Active or irritative hyperemia.
2. Elasticity and its importance as regards the rapidity and uniformity
of the current of blood.—Dilatation of the vessels.
3. Permeability.—Diffusion.—Specific affinities.—Relation between the
supply of blood and nutrition.—Glandular secretion (liver).—Specific action of the elements of the tissues.
Dyscrasia.—Its transitory character and local origin.—Dyscrasia of drunkards. —Haemorrhagic diathesis.—Syphilis.
I Have endeavoured, gentlemen, in the last two lectures to present to you a somewhat detailed picture of the more delicate arrangements which prevail in the body for the conveyance of the different currents of nutritive juices, and particularly for the conveyance of those currents in which the juices themselves are more hidden from observation. Allow me to-day to pass on to the consideration of the larger channels and nobler juices, which, according to prevailing opinion, stand more in the foreground.
The distribution of the blood takes place, as is well known, within the vessels in the following manner: The arteries divide into finer and finer branches, and whilst they thus divide, the character of their walls gradually undergoes such alterations, that at last minute canals, the so-called capillary vessels, appear, provided with a membrane as simple as any that is ever met with in the body. The histological appearances which present themselves in these different vessels are as follows:
On isolating an artery we find that its walls are relatively very thick, and in those arteries which can be followed with the naked eye, not only the well-known three coats are distinguished with the help of the microscope, but in addition to these a fine epithelial layer, which invests the internal surface and is not wont to be included in the class of structures usually termed coats. The internal and external coats are essentially formations of connective tissue, which in the larger arteries display a continually increasing quantity of elastic fibres ; between them lies the relatively thick middle, or circular-fibre, coat, which, as being the seat of the muscular fibres, constitutes what may be almost termed the most important component of the arterial walls. These muscular fibres are found in the greatest abundance in the middle-sized and smaller arteries, whilst in the very large ones, and especially in the aorta, elastic layers form the predominant constituent even of the circular-fibre coat. In small arteries on microscopical examination there may be easily observed within this coat (Comp. Figs. 26, b, b; 45, a) little transverse striations, corresponding to the individual fibre-cells, and encircling the vessel in such dense array, that we find fibre-cell by the side of fibre-cell without any interruption. The thickness of this layer can be readily estimated in consequence of the well-marked limits set to it upon the in- and out-side by the longitudinal-fibre coats; the only deceptive appearance is presented by certain round bodies, which are to be seen here and there in the substance of the circular-fibre coat, but only at the border of the
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 costs 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. M, 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 CAPILLARIES AND SMALL VEINS. 113
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