Imagens das páginas


such as scarcely ever occurs in an equally marked manner in the rest of the muscles, a state which is manifestly the expression of a kind of fatigue and exhaustion, and is the longer persistent, the more energetic the stimulus which was applied. In small vessels with few muscular fihres, therefore, it often seems as if the stimuli really induced no contraction, in consequence of the extreme rapidity with which a state of relaxation is seen to set in, continuing for a considerable time, and allowing of an increased influx of blood.

This same condition of relaxation we can experimentally most easily produce by cutting the nerves supplying the vessels of a part, whilst the contraction can be effected to a very great extent by submitting these nerves to a very energetic stimulus. That our acquaintance with this kind of contraction is of so late a date, is explained by the fact that the stimuli applied to the nerves must be very powerful, and that, as Claude Bernard has shown, only strong electrical currents are sufficient for the purpose. On the other hand, the conditions which ensue upon the section of the nerves are in most parts so complicated, that the dilatation escaped observation, until the lucky spot was discovered also by Bernard, and by the section of the sympathetic nerves in the neck a reliable and convenient field for observation was thrown open to experiment.

We obtain therefore the important fact that, whether the widening of the vessel, or, in other words, the relaxation of its muscular fibres, be produced directly by a paralysis of the nerve or an interruption of the nervous influence, or whether it be the indirect result of a previous stimulation, giving rise to exhaustion—that, I say, in every case we have to deal with a kind of paralysis of the walls of the vessel, and that the process is incorrectly designated active hyperemia, inasmuch as the condition of the vessels in it is always a completely passive one. All that has been built up upon this assumed activity of the vessels, is, if not exactly built upon sand, still of an extremely ambiguous nature, and all the conclusions that have besides been drawn with regard to the important influence which the activity of the vessels was supposed to have upon the conditions of nutrition of the parts themselves, fall at the same time to the ground.

When an artery is really in action, it gives rise to no hyperaemia; the more powerfully it acts, the more does it occasion anaemia, or, as I have designated it, ischamia, and the less or greater degree of activity in the artery determines the greater or less quantity of blood which in a unit of time can stream into a given part. The more active the vessel, the less the supply of blood. If then we have to deal with an hyperaemia the result of irritation, the most important point, therapeutically, is just this, to place the vessels in such a state of activity as will enable them to offer resistance to the onward rush of blood. This we can accomplish by the means of what is called counter-irritation, a higher degree of irritation in an already irritated part, stimulating the fatigued muscular fibres of the vessel to persistent contraction, and thereby diminishing the supply of blood and leading the way to a regulation of the disturbance. In the very cases in which reaction, that is, regulatory activity, is most called for, the chief point is to overcome that state of passiveness which maintains the (so-called active) hyperaemia.

If we now pass from the muscular to the elastic constituents of the vessels, we meet with a property which is of very great importance, on the one hand in the veins, the activity of which is in many cases to be wholly referred to their elastic elements, on the other hand in the arteries and particularly in the aorta and its larger branches. In these the elasticity of the walls has the effect of compensating for the loss which the pressure of the blood experiences from the systolic dilaELASTICITY 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 ectasia, 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 perINFLUENCE 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," 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 hyperaemia, so that the idea readily presents itself

« AnteriorContinuar »