Some are inclined to consider the minutest or proper capillary vessels as destitute of vas cular parietes, and consisting of mere passages through the texture of the organ in which they exist without any lining membrane. This opinion is founded on the impossibility of seeing the coats of the vessels, the rapidity with which new capillaries may be developed, and some other circumstances. The extreme degree of minuteness of the smallest capil lary vessels must render futile any attempts to decide this question by direct observa tion. Besides the general analogy between the larger and smaller vessels, there are several circumstances known which seem to be strongly in favour of the view that the capil laries do not differ in this respect from other vessels. 1st, It is allowable to suppose that the active properties of the capillary vessels belong to parieties as in the larger vessels. 2d, In many transparent parts of animals in which the terminal arteries and veins do not diminish to a very small size, the coats of the vessel may be seen with the microscope, as in the external gills of the Amphibia, and in the vascular rete of the ear of birds and reptiles, in which the capillary vessels may, after having been injected, be separated from the neighbouring soft texture. 3d, The conver sion of small into larger vessels with visible coats in those instances in which the course of blood through the vessels of a part has un dergone an alteration, is in favour of the pre vious existence of parietes in the smaller vessels. And 4th, The constant and regular distribution of the minutest vessels in many parts of animals appears to support the same view. The argument in favour of the non existence of capillary parietes deduced from the alleged facility with which the blood occa sionally passes out of the regular vessels and takes an irregular and indeterminate course through the non-vascular parenchyma of an organ, we believe to be founded, in some in stances, in peculiarities belonging to a few parts only, and in others in inaccurate observation; for in almost all those situations in which the capillary circulation may be seen with ease and distinctness, the constancy of the minute passages which the blood permeates is un doubted.
From the more accurate means of making minute anatomical researches that have been introduced in modern times, the existence of serous, exhalent, and white vessels has become a matter of great doubt, for vessels of this description which do not admit the red glo bules and liquor sanguinis together cannot be made obvious to the senses by the most de licate injections or dissections ; and the ob servation of the capillary circulation in the transparent parts of animals affords the most convincing proof that the smaller arteries have no visible terminations excepting in the capillaries and small veins. In observing attentively the web of the frog's foot and other transparent parts in which the motion of the blood is easily seen, we occasionally see glo bules of blood run into passages of the tissue which we did not perceive before ; but a suf ficient acquaintance with the structure and dis tribution of the smallest of the capillaries in these situations will soon convince the careful observer that the vessels into which the blood was seen to pass, apparently for the first time, existed fully formed before, that the fluid part of the blood passed in part through them, and that the stoppage of the red particles was to a great measure dependent Oil partial or local impediments. The compression of one of the small arteries, for instance, will frequently, after causing oscillation of the globules of the blood in the smallest capillaries, be followed by the disappearance of some of them ; but in a very short time, or when the obstruction is removed, the blood regains its former velo city and force, and flows into exactly the same passages as before.
The notion that the smaller vessels are con tinuous with the smaller lymphatics, and more especially with the excretory ducts of glands, seems to be fully disproved by the accurate researches of Malpighi, Mascagni, Panizza, Muller, and Weber, which have shewn that the lymphatic vessels originate at all parts of the body by a plexus of tubes every where closed, and that the excretory ducts of secre tory organs begin always by shut ends.
We believe it to be satisfactorily shewn that in the whitest of the textures (with the excep tion perhaps of the cornea and crystalline lens), there is DO necessity for the supposition of vessels admitting the fluid parts only of the blood, or of serous vessels, as they have been termed; and that in all of them there exist small bloodvessels which admit very fine rows of globules in their accustomed proportion to the fluid part of the blood : for many textures which appear perfectly white or colourless, or only slightly yellow when viewed with the naked eye, are found, when examined with the inicroscope, to have small vessels carrying blood globules through them. Spallanzani and others
shewed that very small vessels taken singly or seen in very thin layers have almost no per ceptible colour ; and it is a well known fact that, in what are called the red textures, the colour (as of muscle for instance) is not ex clusively dependent upon the quantity of red blood in them. It is difficult, indeed, to con ceive how the circulation of the blood could be carried on at all, or how the red particles of the blood could ever be returned to the heart were the globules to be retained in the larger vessels, and all the white textures to admit only the fluid parts of the blood.
In adopting the opinion that the arteries terminate always by direct continuity of tube in the veins, and that no other visible passages are connected with the minute vessels, we must suppose that the various interchanges of materials occurring between the blood and the organized textures or foreign matters, as in nu trition, secretion, respiration, transpiration, &c. must take place by some process of organic transudation through invisible apertures of the minute vessels.
b. Properties of the capillary vessels and in fluence on the circulation.—From the expe riments already referred to, it is apparent that the smaller arteries, so long as they can be distinguished from other vessels, are capable of being excited to contraction by the appli cation of a stimulus ; but we have no means of sliewing this with regard to the minutest capillary vessels, because we can scarcely apply any stimulation to them without affectin soMe of the smaller arteries at the same time. When it is said, for example, that the capillary ves sels are irritable, because the application of ammonia or spirits of wine causes them to become smaller, it is difficult to determine how far this appearance of diminished size in the capillaries depends on their receiving less blood, in consequence of the contraction of the small arteries leading to them or upon the less size of these vessels themselves. In the expe riments of Dr. Thomson and others, however, the application of salt and other stimuli ex citing inflammation have appeared to dilate even the smallest capillary vessels, and such a dilatation can scarcely be considered as in dicating any thing else than a less power of resistance in these vessels ; and when the ap plication of ammonia or spirit of wine restores such dilated capillaries to their natural con= dition, we do not see that any other natural inference can be drawn from this fact than that the capillaries have been contracted by the influence of these stimuli ; for the contraction of the small arteries alone, although it might restore the lost velocity of the blood, would not diminish the capillaries to their former size. This general diminution of size ought how ever to be carefully distinguished from the more marked and local contractions of true arteries. • The velocity of the blood is quite uniform in the capillaries of the adult animal in the natural condition of the circulation. There is reason to believe the capillary vessels to be highly elastic, and to have the effect of com pleting the change which is begun by the arteries, viz. that of equalizing the force of the heart transmitted through the blood. We do not, in observing attentively the capillary 'vessels, ever perceive any motions of alter nate dilatation •and contraction of their sides. The blood flows through them as through small glass tubes ; and if they act by other powers than by their elasticity alone, this action must be of so slow a kind as not to be perceptible. There can be no doubt that any action of contraction occurring in the capillary vessels, whether alternating with dilatation or not, could have no effect excepting that of ob structing the passage of blood through them. It would act upon the contents of the arterial system much in the same way as the dimi nution of the aperture at the end of a rigid tube would affect the flow of fluid through it, that is, either a less quantity of blood would pass through the capillary vessels in conse quence of their less size, or a greater portion of the heart's force would be. expended in di lating these vessels to a sufficient extent.