ELASTICITY OF THE COATS OF VESSELS. 121 tation of the vessels, and of converting the uneven current produced by the jerking movements of the heart into an even one. If the walls of the vessels were not elastic, the stream of the blood would unquestionably be rendered very much slower, and at the same time pulsation would take place throughout the whole extent of the vascular apparatus as far as the capillaries, for the same jerking movement which is communicated to the blood at the commencement of the aortic system would continue even into the smallest ramifications. But every observation we make in living animals teaches us that within the capillaries the stream is a continuous one. This equable onward movement is effected by the elasticity of the walls of the arteries, in virtue of which they return the impulse which they receive from the in-rushing blood with the same force, and by this means maintain a regular onflow of the blood during the time occupied by the following diastole of the heart. If the elasticity of the vessel be considerably diminished, without its becoming stiff and immoveable (from calcareous incrustations) in the same degree, the dilatation which it undergoes from the pressure of the blood, is not again compensated; the vessel remains in a dilated condition, and thus are gradually produced the well-known forms of ectasis, such as we are familiar with in the arteries under the name of aneurysms, and in the veins under that of varices. In these processes we have not so much, as has been represented of late, to deal with primary disease of the inner coat, as with changes which are situated in the elastic and muscular middle coat. If therefore it is the muscular elements of the arteries that have the most important influence upon the quantity of blood to be distributed, and the mode of its distribution, in the several organs, and the elastic elements that are chiefly concerned in the production of a rapid and equable stream, they nevertheless exercise only an indirect influence upon the nutrition of the parts which lie outside the vessels themselves, and in this matter, we are obliged to betake ourselves, as a last resource, to the simple, homogeneous membrane of the capillaries, without which indeed not even the constituents of the walls of the larger vessels provided with vasa vasorum would be able to maintain themselves for any lengthened period. The difficulty which here presents itself has, as you know, during the last ten years, been chiefly got over by the assumption of the existence of diffusive currents (endosmosis and exosmosis) between the contents of the vessels and the fluid in the tissues; and by regarding the capillary wall as a more or less indifferent membrane, forming merely a partition between two fluids, which enter into a reciprocal relation with one another; while the nature of this relation would be essentially determined by the state of concentration they are in and their chemical composition, so that, according as the internal or the external fluid was the more concentrated, the diffusive stream would run inwardly or outwardly, and, according to the chemical peculiarities of the individual juices, certain modifications would arise in these currents. Generally speaking, however, the chemical side of this question has been but little regarded. It cannot be denied that there are certain facts which cannot well be explained in any other manner, especially in cases where essential alterations have taken place in the state of concentration of the juices, for example, in that form of cataract which Kunde has artificially produced in frogs by the introduction of salt into their intestinal canal or subcutaneous cellular tissue. But in proportion as, after a physical study of the phenomena of diffusion, the conviction has been acquired that the membrane which separates the fluids is not an indifferent substance, but that its nature exercises a directly controlling influence upon the per INFLUENCE OF THE VESSELS UPON NUTRITION. 123 meating powers of the fluids, it becomes impossible that a like influence should be denied the capillary membrane. We must not, however, go so far, as to ascribe to this membrane all the peculiarities observable in the interchange of material, and so explain how it happens, that certain matters, which enter into the composition of the blood are not distributed in equal proportion to every part, but leave the vessels at some points in greater, at others in less, quantity, and at others not at all. These peculiarities depend manifestly, on the one hand, upon the different degrees of pressure to which the column of blood is subjected in certain parts, and, on the other, upon special properties of the tissues; and we are irresistibly compelled both by the consideration of simply pathological, and particularly by that of pharmaco-dynamical, phenomena to admit that there are certain affinities existing between definite tissues and definite substances, which must be referred to peculiarities of chemical constitution, in virtue of which certain parts are enabled in a greater degree than others to attract certain substances from the neighbouring blood. If we consider the possibility of such attraction with a little more attention, it is peculiarly interesting to observe the behaviour of parts, which are at a certain distance from the vessel. If we apply a definite stimulus, for example, a chemical substance, a small quantity of an alkali I will suppose, directly to any part, we see that this shortly afterwards takes up more nutritive matter, so that even in a few hours its size becomes considerably increased, and that, whilst before we were perhaps scarcely able to distinguish anything in its interior, we now find an abundant, relatively opaque material within it, in no wise composed of alkali which had made its way in, but essentially containing substances of an albuminous nature. Observation shews that the process in all vascular parts begins with hyperæmia, so that the idea readily presents itself cause. that the hyperæmia is the essential and determining But if we investigate the matter more minutely, we find it difficult to understand how the blood, which is in the hyperæmic vessels, can contrive only to act just upon the irritated part, whilst other parts lying in the immediate vicinity are not affected in the same manner. In all cases in which the vessels are the immediate originators of disturbances which take place in a tissue, these are most marked in the immediate neighbourhood of the vessels and in the district which they supply (vascular (or vessel-) territory). If we introduce an irritating, as for example a decomposing, body into a blood-vessel, a fact that has been established by me upon a large scale when tracing out the history of embolia, we by no means see that the parts at a distance from the vessel are the principal seats of active change, but that this is in the first instance manifested in the wall of the vessel itself and then in the adjoining histological elements. But if we apply the stimulus directly to the tissue, the central point of the disturbance will always continue to be that at which the stimulus produced its effect, just the same whether there are vessels in the neighbourhood or not. We shall hereafter have occasion to return to this subject, and my only object here was to lay this fact before you in its general features, and thus repel the ordinary conclusion, which is as convenient as it is fallacious, that hyperæmia (in itself passive) exercises a directly regulating influence upon the nutrition of tissues. If a special proof were still required in order to complete the refutation of this assumption, which in an anatomical point of view is utterly untenable, we find a most apposite argument in the experiment above mentioned of the section of the sympathetic. In an animal the sympathetic may be divided in the neck; thereupon a state of hyperæmia ensues in the whole of that half of the head, the ears become dark INFLUENCE OF THE VESSELS UPON NUTRITION. 125 red, the vessels greatly dilated, the conjunctiva and nasal mucous membrane turgidly injected. This may continue for days, weeks, or months, without the least appreciable nutritive disturbance necessarily arising therefrom; the parts, although gorged with blood, are yet, as far at least as we are at present able to judge of this, in the same state of nutrition as before. If we apply stimuli sufficient to produce inflammation to these parts, the only thing that we remark is, that the inflammation runs its course more quickly, without exhibiting either in itself or in the nature of its products anything essentially unusual. The greater or less quantity of blood, therefore, which flows through a part is not to be regarded as the only cause of the changes in its nutrition. There is, I suppose, no doubt that, when a part, which is in a state of irritation, receives more blood than usual, it is also able to attract a larger quantity of material from the blood with greater readiness than it otherwise could have done, or than it would be able to do, if the vessels were in a state of contraction or less filled with blood. If therefore it were to be objected to my view, that in such conditions the most favorable effects are often produced by local abstractions of blood, this would be no proof of its incorrectness. If we cut off or diminish the supply of nutritive matter, we must of course prevent the part from absorbing more than its wont, but vice versa we cannot by offering it a larger quantity of nutritive material straightway compel it to take up more than it did; these are two entirely independent cases. However apt one may be to conclude (and however much I may be disposed to allow, that at the first glance there is something very plausible in such a conclusion) that, from the favorable effect which the cutting off of the supply of blood has in putting a stop to a process which arose from an increase of it, the process depended this increased supply, yet I am of opinion that the upon |