Bleeding from small arteries that have been wounded is arrested, to a large extent, by the contraction of the muscular coats of the vessels. Tincture of steel and such substances, when applied to bleeding surfaces, stop the flow of blood because they excite the muscular fibres to contract, and so shut the mouths of the open vessels. It is for the same reason that tannic and gallic acids, &c., are ordered for bleeding from internal surfaces. To diminish and arrest internal bleeding a drug called ergot of rye, or spurred rye, is frequently used with excellent results. Now this drug does not act directly on the bleeding part, but it enters the blood and produces contraction of the muscular coat of the vessels. It arrests bleeding by stimulating the contractility of the smaller arteries.
The Circulation in the Capillaries must be considered next, since the blood flows from arteries into capillaries. The best way to ob serve the circulation in the capillaries is to fix up a living frog in such a way that the web of its foot is stretched so as to be viewed through a microscope. A fine transparent tissue is seen with clumps of black colouring matter here and there. The tissue is cut up by Thannels per vading it in all directions, some wide, some ex tremely narrow. Along these channels blood is streaming. In the wide vessels it is dashing along with apparently great speed, the red cor puscles streaming in crowds down the centre, while the white corpuscles are seen rolling along nearer to the walls of the vessel; in the nar rowest vessels there is evidently room for only one corpuscle at a time, and the corpuscles range themselves along, accommodating their shape to the turns and corners so as not to stick in the channel. The noticeable feature is that the current is continuous. When it is remembered that the flow in the arteries is jerky, and that the capillaries are continuous with the arteries, this seems strange. Why is it that the jerking movement of the blood in the arteries is not continued into the cap illaries? The explanation is found in the elas ticity of the arteries. Were the blood-vessels rigid tubes, then there would be ejected from one end just the quantity of fluid forced in at the other, and in the same intermittent way. The blood encounters resistance in the capillar ies, it cannot get along so quickly as in the arteries, the arteries are, therefore, kept con stantly distended, and their elastic force is brought into full play. During the pause after
each contraction of the heart the elastic walls of the arteries are pressing on the blood they contain, and are following up the force of the heart. It is as if there were two propelling forces, the one following the other so smoothly and regularly that there is no stoppage in the onward movement. If the elastic tube is long enough the result is that, by and by, the wave like movement of the fluid in it becomes less and less perceptible, till it finally disappears, and the intermittent movement is converted into a continuous flow. It must be observed, however, that it is only where there is sufficient resistance to the progress of the fluid that this can happen, for if there is little resistance the elastic reaction is not brought into play, and the jerky movement continues. The next thing noticeable, connected with the capillaries, is the thin walls of the vessels, so that the oozing of fluid, already referred to, for purposes of nourishing the tissues, is easily understood. Another feature that one remarks is that every minute part of tissue is so surrounded with capillaries that it cannot fail to receive nourish ment from all sides.
Generally speaking, every organ and tissue of the body abounds in capillary blood-vessels, even bone being interpenetrated by them. Tissues so traversed by vessels are said to be vascular. There are, however, a few tissues which are non-vascular, that is, they have no such system of vessels within them. These are epidermis and epithelium, in other words, the surface layer of the skin and mucous membrane; and nail, hair, the substance of the teeth, and carti lage or gristle. Such tissues, however, are closely connected with moist vascular tissues, from which they are able to suck up nourishment.
The Circulation in the Veins is charac terized by a continuous flow, the wavelike movement having been completely lost in the capillaries. Besides the force from behind— the force that is transmitted through the capillaries from the arteries—other agents enter to impel the blood in its progress through veins. Chief of these are the valves which are present, with exceptions, in veins, and permit the flow of blood towards the heart, but close if there is any attempt at a backward flow. Contracting muscles, by pressing on veins, and, consequently, on the blood they contain, aid in the venous part of the circulation.