BLOOD-VESSELS.

The blood, from which the solid textures immediately derive material for their nourishment, is conveyed through the body by branched tubes named blood-vessels. It is driven along these channels by the action of the heart, which is a hollow muscular organ placed in the centre of the sanguiferous system. One set of vessels, the arteries, conducts the blood out from the heart and distributes it to the different regions of the body, whilst other vessels, the veins, bring it back to the heart again. From the extreme branches of the arteries the blood gets into the commencing branches of the veins or revehent vessels, by passing through a network of fine tubes which connect the two, and which are termed, by reason of their smallness, the capillary (ie., hair-like) vessels, or, simply, the capillaries.

ARTERIES.

These vessels were originally supposed to contain air. This error, which had long prevailed in the schools of medicine, was refuted by Galen, who showed that the vessels called arteries, though for the most part found empty after death, really contain blood in the living body.

Mode of distribution.-The arteries usually occupy protected situations; thus, after coming out of the great visceral cavities of the body, they run along the limbs on the aspect of flexion, and not upon that of extension where they would be more exposed to accidental injury.

As they proceed in their course the arteries divide into branches, and the division may take place in different modes. An artery may at once resolve itself into two or more branches, no one of which greatly exceeds the rest in magnitude, or it may give off several branches in succession and still maintain its character as a trunk. The branches come off at different angles, most commonly so as to form an acute angle with the further part of the trunk, but sometimes a right or an obtuse angle, of which there are examples in the origin of the intercostal arteries.

An artery, after a branch has gone off from it, is smaller than before, but usually continues uniform in diameter or cylindrical until the next secession; thus it was found by Hunter that the long carotid artery of the camel does not diminish in calibre throughout its ler-gth. A branch of an artery is less than the trunk from which it springs, but the combined area or collective capacity of all the branches into which an artery divides, is greater than the calibre of the parent vessel immediately above the point of division. The increase in the joint capacity of the branches over that of the trunk is not in the same proportion in every instance of division, and there is at least one case known in which there is no enlargement, namely, the division of the aorta into the common iliac and sacral arteries; still, notwithstanding this and other possible exceptions, it must be admitted as a general rule that an enlargement of area takes place. From this it is plain that, since the area of the arterial system increases as its vessels divide, the capacity of the smallest vessels and capillaries will be greatest; and, as the same rule applies to the veins, it follows that the arterial and venous systems may be represented, as regards capacity, by two cones whose apices (truncated it is true) are at the heart, and whose bases are united in the capillary system. The effect of this must be to make the blood move more slowly as it advances along the arteries to the capillaries, like the current of a river when it flows in a wider and deeper channel, and to accelerate its speed as it returns from the capillaries to the venous trunks.

When arteries unite they are said to anastomose or inosculate. Anastomoses may occur in tolerably large arteries, as those at the base of the brain, those of the hand and foot, and the mesentery, but they are much more frequent in the smaller vessels. Such inosculations admit of a free communication between the currents of blood, and must tend to promote equability of distribution and of pressure, and to obviate the effects of local interruption.

Arteries commonly pursue a tolerably straight course, but in some parts they are tortuous. Examples of this in the human body are afforded by the arteries of the lips and of the uterus, but more striking instances may be seen in some of the lower animals, as in the well-known case of the long and tortuous spermatic arteries of the ram and the bull. In very moveable parts like the lips, this tortuosity will allow the vessel to follow their motions without undue stretching; but in other cases its purpose is not clear. The physical effect of such a condition of the vessel on the blood flowing along it must be to reduce the velocity, by increasing the extent of surface over which the blood moves, and consequently the amount of impediment from friction; still it does not satisfactorily appear why such an end should be provided for in the several cases in which arteries are known to follow a tortuous course. The same remark applies to the peculiar arrangement of vessels named a "rete mirabile," where an artery suddenly divides into small anastomosing branches, which in many cases unite again to re-construct and continue the trunk. Of such retia mirabilia there are many examples in the lower animals, but, as already remarked, the purpose which they serve is not apparent. The best known instance is that named the rete mirabile of Galen, which is formed by the intracranial part of the internal carotid artery of the sheep and several other quadrupeds.

Arteries possess considerable strength and a very high degree of elasticity, being extensible and retractile both in their length and their width. When cut across they present, although empty, an open orifice; the veins, on the other hand, collapse, unless when prevented by connection with surrounding rigid parts.

Structure. In most parts of the body the arteries are inclosed in a sheath formed of connective tissue, and their outer coat is connected to the sheath by filaments of the same tissue, but so loosely that, when the vessel is cut across, its ends

Fig. 417.-TRANSVERSE SECTION OF PART OF THE WALL OF THE POSTERIOR TIBIAL ARTERY (MAN). 75 DIAMETERS. (E. A. S.)

a, epithelial (endothelial) and subepithelial layers of inner coat; b, elastic layer (fenestrated membrane), of inner coat, appearing as a bright line in section; c, muscular layer (middle coat); d, outer coat, consisting of connective tissue bundles. In the interstices of the bundles are some connective tissue nuclei, and, especially near the muscular coat, a number of elastic fibres cut across.

readily shrink some way within the sheath. Some arteries lack sheaths, those for example which are situated within the cavity of the cranium.

Independently of this sheath, arteries (except those of minute size whose structure will be afterwards noticed) have been usually described as formed of three coats, named, from their relative position, internal, middle, and external (fig. 417, in section); and as this nomenclature is generally followed in medical and surgical works, and also correctly applies to the structure of arteries so far as it is discernible

by the naked eye, it seems best to adhere to it as the basis of our description; although it will be seen, as we proceed, that some of these coats are found on microscopic examination really to consist of two or more strata differing from each other in texture, and therefore reckoned as so many distinct coats by some authorities.

Internal coat (Tunica intima) (fig. 417, a, b). This may be raised from the inner surface of the arteries as a fine transparent colourless membrane, elastic but very easily broken, especially in the circular or transverse direction, so that it cannot be stripped off in large pieces. It is very commonly corrugated with fine and close longitudinal wrinkles, caused most, probably by a contracted state of the artery after death. Such is the appearance presented by the internal coat to the naked eye, but by the aid of the microscope, it is found to consist of three different structures, namely:

1. An epithelial layer (endothelium of the artery) (fig. 417, a, and fig. 418) forming the innermost part or lining. This is a simple layer of thin elliptical or irregularly polygonal cells, which are often lengthened into a lanceolate shape. The cells have round or oval nuclei, with nucleoli: their outlines are often indistinct in

Fig. 418.-EPITHELIAL LAYER LINING THE POSTERIOR TIBIAL ARTERY OF MAN. 250 DIAMETERS.

(E. A. S.)

Nitrate of silver preparation.

Fig. 419. CELL SPACES OF SUB-EPITHELIAL LAYER OF ARTERY (POSTERIOR TIBIAL). 250 DIAMETERS.

(E. A. S.)

The ground substance is stained by nitrate of silver, and the cell-spaces of the tissue are thus made manifest as white patches, the contained cells not being seen.

the fresh state, but may be brought into view by means of nitrate of silver. When the vessels are empty and collapsed, the endothelium cells are less flattened, and the part of each cell which contains the nucleus may project somewhat into the lumen of the vessel.

2. A subepithelial layer (striated layer of Kölliker). This is composed of a finely fibrillated connective tissue with a number of branched corpuscles lying in the cell-spaces of the tissue (fig. 419). This layer is most developed in the larger arteries it exists however as a thin stratum in the medium-sized ones. In the aorta it is very well marked and contains a large number of anastomosing cells and cell-spaces lying in a finely fibrillated ground-substance. Longitudinal networks of

:

very fine elastic fibres, which are in continuity with the larger elastic fibres of the next layer, occur in it in the aorta.

3. Elastic layer (fig. 417, b). The chief substance of the inner coat is formed. by elastic tissue, which occurs as longitudinal networks of fibres (fig. 420), consisting of one or more layers of different degrees of closeness. Not uncommonly some of these (or one in particular) take on a membranous character, in which case the "perforated" or "fenestrated" membrane of Henle is formed. This consists of a

thin and brittle transparent film of elastic tissue. It can be stripped off in small shreds, which have a remarkable tendency to curl in at their borders, and roll themselves up as represented in fig. 421. The films of membrane are marked by fine lines, following principally a longitudinal direction, and joining each other obliquely in a sort of network. These lines are reticulating fibres formed upon the membranous layer and continuous with the reticulating elastic fibres which pervade the muscular coat on the one side and with those which extend into the subepithelial

Fig. 421.-PORTION OF FENESTRATED MEMBRANE FROM THE FEMORAL ARTERY, MAGNIFIED 200 DIAMETERS. (Henle.) a, b, c, perforations.

layer on the other. The membrane is further remarkable by being perforated with numerous round, oval, or irregularly shaped apertures of different sizes. In some parts of the arteries the perforated membrane is very thin, and therefore difficult to strip off; in other situations it is of considerable thickness, consisting of several layers; in which case it tends in the outer layers to lose its membranous character: indeed it must be borne in mind that every transition is met with between the fenestrated membranes, and the longitudinal elastic network.

The inner coat in its most developed condition may thus be said to be formed of (1) a layer of flattened epithelial cells (endothelium), (2) a layer of delicate connective tissue with branched cells; and (3) of elastic tissue under two principal forms, namely, the longitudinal elastic networks and the fenestrated membrane; and these two forms may coexist in equal amount, or one may predominate, the other diminishing or even disappearing altogether.

Middle coat (Tunica media) (fig. 417, c). This consists of plain muscular tissue, in fine bundles, disposed circularly round the vessel, and consequently tearing off in a circular direction, although the individual bundles do not form complete rings. The considerable thickness of the walls of the arteries is due chiefly to this coat; in the smaller ones, it is thicker in comparison with the calibre of the vessel. In the larger vessels it is made up of many layers; and elastic films either finely reticular, or quite similar to the fenestrated membrane of the inner coat, are found between the muscular layers and alternating with them, being also united with one

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another by elastic fibres passing across the muscular bundles. In most arteries this elastic tissue of the middle coat is but slightly developed, but in the aorta (fig. 424) and carotid arteries and in some of the branches of the latter, it attains a considerable development, and since in them also elastic fibres are seen extending into the subepithelial layer of the inner coat, the distinction between the inner and middle coats as shown in section is far less marked than it is in ordinary arteries. There is also a not inconsiderable amount of connective tissue in the middle coat of the aorta.

The muscular fibre-cells of the middle coat of the arteries (fig. 422 and fig. 423) are seldom more than from to of an inch long and frequently, especially in

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a

Fig. 422.-MUSCULAR FIBRE-CELLS FROM HUMAN ARTERIES. MAGNIFIED 350 DIAMETERS.

1. From the popliteal artery; a, natural; b, treated with acetic acid. 2. From a small branch of the posterior tibial (from Kölliker).

Fig. 423.-MUSCULAR FIBRE-CELLS FROM SUPERIOR THYROID ARTERY (MAN). 340 DIAMETERS. (E. A. S.)

those arteries in which the elastic tissue of the middle coat is most developed, present a very irregular shape with jagged extremities (fig. 423). Their nuclei are distinctly rod-shaped and are often slightly curved.

Bundles of white connective-tissue fibrils may also occur in small quantity in the middle coat, the proportion increasing with the size of the artery. It is important to note that the muscular tissue of the middle coat is more pure in the smaller arteries, and that the admixture of other tissues increases in the larger-sized vessels; in these, moreover, the muscular cells are smaller. Accordingly, the contractility of the arteries, which depends on the muscular tissue of the middle coat, is little marked in those of large size, but becomes much more conspicuous in the smaller branches.

External coat (Tunica adventitia) (fig. 417, d). This is composed mainly of fine and closely-felted bundles of white connective tissue, together with a variable amount of longitudinally disposed elastic tissue between the bundles (in the figure the elastic fibres are seen cut across). The elastic tissue is much more abundant towards the inner part, next the muscular coat, and is frequently described as constituting here a distinct elastic layer: it is most marked in arteries of medium calibre, becoming thinner, and at length gradually disappearing in those of small size.

In large and middle-sized arteries the bundles of white connective tissue chiefly run diagonally or obliquely round the vessel, and their interlacement becomes much