CAPILLARY VESSELS, so called from their hair-like minute ness. Tho blood-vessels of the body consist of arteries and veins, the arteries carrying the blood from the heart, and the veins returning it to the heart. The blood-vessels that supply the body are arborescent, that is, the branches which spring from the aorta successively increase in number and diminish in size as they proceed from the heart towards their ultimate terminations in the system. In like manner the veins divide. These ultimate terminations of the arteries, together with the first origins of the veins, constitute a peculiar system of vessels termed the Capillary System. These capillary vessels are too minute to be detected by the naked eye ; but in the transparent parts of the body of a living animal, when brought under the field of the microscope, they become perfectly visible, as in the web of the frog's foot, the mesentery of the rabbit, the tail of the tadpole, &c. The greater number of the arteries and veins are then seen to be directly continuous with each other, no substance intervening between the two orders of vessels. No words can describe the beauty of the sight presented by the flow of the vital fluid through these minute tubes. Myriads of vessels not visible to the naked eye instantly come into view. In oue case the direction of a minute artery being suddenly altered it is reflected on itself, and thus becomes an incipient vein ; in other cases minute branches are sent off from an artery into a parallel vein ; and in a third case several minute arterial ramifications are continuous with a single vein. The venous capillaries are generally larger and more numerous than the arterial, and they communicate more freely with each other.
The minute capillary vessels are totally distinct both in structure and office from the large trunks from which they spring. All the tunics of the capillary arteries diminish in thickness and strength as the tubes lessen in size, but more especially the middle or fibrous coat [Awrent] ; "but this coat may still be distinguished by its colour in the transverse section of any artery whose internal diameter is not less than the tenth of a line, hut it entirely disappears in vessels too small and too remote to receive the wave of blood in a manifest jet. But while the membranous tunics diminish, the nervous filaments distributed to them increase. The smaller and thinner the capillary the greater the proportionate quantity of its nervous matter; and this is most manifest in organs of the greatest irritability. The coats of the capillaries successively becoming thinner and thinner at length disappear altogether, and the vessels ultimately terminate in membraueless canals formed in the substance of the tissues." • Of the capillary arteries which it has been stated terminate by direct communication with the capillary VeillP, sonic are large enough to admit of three or four of the red particles of the blood [ilioon] abreast ; the diameter of others is sufficient to admit only of one ; whilst others are so small that they can transmit nothing but the serum of the blood. Their prevalent size in the Lumen body may be
stated at from ,ths to of an inch when naturally filled with blood. As long as the capillary is of sufficient magnitude to receive three or four blood globules abreast, it is evident that it possesses regular parietes; but by far the greater number, before they communicate with veins, lose altogether their membranous coats. There are no visible openings or pores in the sides or ends of the capillaries by means of which the blood can be extravasated preparatory to its being imbibed by the veins. There is nowhere apparent a sudden passage of the arterial into the venous stream, no abrupt boundary between the division of the two systems. The arterial streamlet wind. through long routes, and describes numerous turns before it assumes the nature and takes the direction of it venous etreanilet. The ultimate capillary rarely passes from n large arterial into a largo venous branch.
The capillary network differs in the size and width of the meshes in different parts. It is very close in the lungs and in the ehomid coat of the eye; close also in muscle, in the skin, and in most parts of the mucous membrane, in glands and secreting structures, and in the gray part of the brain and spinal cord. On the other hand, it has wide meshes and comparatively few vessels in the ligaments, tendons, and other allied textures. (Sharpey.) All the great organic functions of the living body are performed mainly by the capillary vessels. Their action is essential to secre tion, nutrition, celorification, and every other process which is indispensable to the support of life. From experiment, it has been inferred that these vessels possess an active contractile power altogether independent of the impulse derived from the heart. Under the ordinary condition of the circulation, the blood indeed flows through these capillary vessels by the force communicated to the circulating fluid by the contraction of the heart; but the evidence brought forward seems to indicate that stimulants of various kinds applied directly to the capillary arteries, without in the least affecting the heart's action, are capable of modifying to a considerable extent the action of the capillaries ; sometimes causing them to contract and at other times to dilate; sometimes quickening the flow of the blood through them; at other times retarding it, and not unfrequently altogether arresting its progress.
For an account of the development of the capillaries, see Bsoon VESSELS.