3. Phenomena of the capillary circulation.— The phenomena of the passage of the blood from the terminations of the arteries into the Commencement of the veins through the capil lary vessels, are highly interesting and impor tant in many points of view, for the immediate respiratory change which the venous blood undergoes in the pulmonary vessels, and all those alterations of composition which accom pany nutrition, growth, secretion, and other org,anic processes connected with the systemic vessels, occur in the smallest ramifications of the pulmonic and systemic circulation, and the morbid state of inflammation as well as the various pathological changes which occur as its consequences are intimately connected with an altered condition of the capillary system.
a. Structure and distribution of the capillary vessels.—The name of capillary is generally given to all those minute vessels which form the means of communication between the small ramifications of the arteries and veins; but there is some difference in the opinion of anatomists and physiologists as to how much of the vascular system ought to be included under the division of the capillary vessels. Some, adhering to the strict meaning of the term, apply it to all the small vessels whatso ever under a certain size ; others hold that between the extremities of the arteries and veins there is always situated a series of minute tubes of nearly equal size in their whole length, and not ramifying like the arteries or veins, which constitute a system of vessels distinct from the others in their structure, distribution, and properties, to which the name of capillary ought to be restricted.t The last view appears to us to be founded in a partial acquaintance with the system of minute vessels, for though it may be true that in some parts of animals the capillaries have obviously the structure above described, and seem to form a system of vessels apart from the smaller arteries and veins, yet this is by no means the case in other textures; and we think that the more extensive observation of the structure of these vessels in various parts will shew that in the greater number, as is well ascertained to exist in many, the smaller arteries pass into veins quite in a gradual manner, the ramifications of each class of vessel becoming more and more minute until they meet, the two kinds of vessel presenting no difference of character other than the change of direction assumed by the moving blood, which enables us to say with certainty where the artery termi nates, and at what point the vein begins, and affording thus no reason to consider the continuous tube by which they join as different in structure from either the minute artery or vein. While we acknowledge therefore the
importance of the observations which point out the existence of capillary vessels of a uni form size in some textures, we think it necessary to retain the name of capillary as applied to all the minute vessels, both for the reason that the communicating vessels are not every where of the same kind, and that from the use already made of the term by physiological writers its meaning will thus be more easily understood.
The vessels which lead from arteries to veins are of very various sizes, some admitting only one globule at once, others being so large as to allow of the passage of three, four, or even a greater number of red globules together. In tracing with the microscope the motion of the minute streams of blood as they pass through the capillary vessels, the eye is guided by the motions of the red globules principally, for it is very rarely indeed that the current of fluid which carries the globules along can be recog nized in the ordinary modes of observation.
The capillary circulation is most easily seen in cold-blooded and in young animals, both On account of the large size of the red glo bules and the small number of the vessels. Since the first discovery of the capillary circu lation by Malpighi, the transparent web be tween the toes of the hind feet of the frog has been universally adopted as the most con venient situation for observing this beautiful spectacle with transmitted light. The fins and tail of fishes, the tail of the larva of the Frog and Newt, the external gills of the same ani mals as well as of cartilaginous fishes, the mesentery of the Frog or of small warm blooded animals, the wing of the Bat, the lungs and urinary bladder of Reptiles, the liver of the Frog and Newt, the membranes of the incubated egg, the yolk of the Skate's egg, are all situations favourable for the ob servation of the capillary circulation. The capillary circulation has been viewed in only a small number of warm-blooded animals, and in very few of their textures; but the minute injection with coloured fluids of all parts of the bodies of Quadrupeds and of Man leaves little doubt that in them also, whatever vari eties there may be in the size, number, and distribution of the small vessels, the blood passes in every organ from the small arteries into the returning veins by minute continuous tubes of the same nature as those more easily observed in the situations above-mentioned.