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RESPONSE OF TISSUES TO

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Originally appearing in Volume V20, Page 924 of the 1911 Encyclopedia Britannica.
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RESPONSE OF TISSUES TO STIMULATION A stimulus may be defined as every See also:

change of the See also:external agencies acting upon an organism; and if a stimulus come in contact with a See also:body possessing the See also:property of irritability, i.e. the capability of reacting to stimuli, the result is stimulation (Verworn). Stimuli comprise chemical, See also:mechanical, thermal, photic and See also:electrical changes in the environment of the organism. A stimulus may See also:act on all sides and induce a See also:general effect with-out direction of See also:movement, but in the See also:production of movement in a definite direction the stimulus must be applied unilaterally. Stimuli applied generally, not unilaterally, in most cases induce increased divisibility of the cells of the See also:part. Thus the See also:poison of various See also:insects induces in See also:plants the cellular new formation known as a See also:gall-See also:nut; a See also:foreign body implanted in a See also:limb may become encysted in a See also:capsule of fibrous See also:tissue; septic See also:matter introduced into the See also:abdomen will cause proliferation of the lining endo(epi)thelium; and placing an See also:animal (See also:salamander, Galeotti) in an See also:ambient See also:medium at a higher temperature than that to which it is accustomed naturally, increases the rapidity of See also:cell-See also:division of its epithelium with See also:augmentation of the number of karyokinetic figures. See also:Hair and some other like structures grow luxuriantly on a part to which there is an excessive See also:flux of See also:blood. See also:Bone (e.g. See also:drill-bones) may develop in a soft tissue with no natural bone-forming tendencies, as a result of interrupted pressure, or a fatty See also:tumour may arise in the midst of the natural subcutaneous See also:fat in the same circumstances. Among stimuli acting unilaterally, perhaps none has proved more interesting, in See also:late times, than what is known as Chemiotaxis. By it is meant the property an organism endowed with the See also:power of movement has to move towards or away from a chemical stimulus applied unilaterally, or, at any See also:rate, where it is applied in a more concentrated See also:state on the one See also:side than on the others, and more particularly where the concentration increases gradually in one direction away from the living organism acted upon. Observed originally by Engelmann in bacteria, by See also:Stahl in myxomycetes, and by Pfeffer in ferns, mosses, &c., it has now become recognized as a widespread phenomenon. The See also:influence of the chemical substance is either that of attraction or repulsion, the one being known as ,posita`es the other as negative chemiotaxis. The See also:female See also:organs of certain cryptogams, for instance, exert a See also:positive chemiotactic See also:action upon the spermatozoids, and probably, as Pfeffer suggests, the chemical See also:agent which exerts the influence is malic See also:acid.

No other substance, at least, with which he experimented had a like effect, and it is possible that in the archegonium which contains the ovum malic acid is See also:

present. Massart and Bordet, See also:Leber, Metchnikoff and others have studied the phenomenon in leucocytes, with the result that while there is See also:evidence of their being positively chemiotactic to the toxins of many pathogenic microbes, it is also apparent that they are negatively influenced by such substances as lactic acid. From a pathological point of view the subject of chemiotaxis must be considered along with that of See also:phagocytosis. Certain See also:free See also:mobile cells within the body, such as blood-leucocytes, as well as others which are fixed, as for instance the endothelium of the hepatic capillaries, have the property of seizing upon some kinds of particulate matter brought within their reach. Within a See also:quarter of an See also:hour after a quantity of See also:cinnabar has been injected into the blood of the See also:frog nearly every particle will be found engulfed by the See also:protoplasm of the leucocytes of the circulating blood. Some bacteria, such as those of See also:anthrax, are seized upon in the same manner, indeed; very much as small See also:algae and other particles are incorporated and devoured by See also:amoeba. Melanine particles formed in the See also:spleen in See also:malaria, which pass along with the blood through the See also:liver, are appropriated by the endothelial cells of the hepatic capillaries, and are found embedded within their substance. If the particle enveloped by the protoplasm be of an organic nature, such as a bacterium, it undergoes digestion, and ultimately becomes destroyed, and accordingly the See also:term " phagocyte " is now in See also:common use to indicate cells having the above properties. This phagocytal action of certain cells of the body is held by Metchnikoff and his followers tohave an important bearing on the See also:pathology of See also:immunity. Phagocytes act as scavengers in See also:ridding the body of noxious particles, and more especially of harmful bacteria. A further application of the facts of chemiotaxis and phagocytosis has been made by Metchnikoff to the See also:case of Inflammation. It is well known that many attempts to define the See also:process of inflammation have been made from See also:time to time, all of them more or less unsatisfactory.

Among the Iatest is that of Metchnikoff: " Inflammation generally," he says, " must be regarded as a phagocytic reaction on the part of the organism against irritants. This reaction. is carried out by the mobile phagocytes sometimes alone, sometimes with the aid of the vascular phagocytes, or of the See also:

nervous See also:system." Given a noxious agent in a tissue, such, let us say, as a localized See also:deposit of certain bacteria, the phagocytes swarm towards the locality where the bacteria have taken up their See also:residence. They surround individual bacteria, absorb them into their substance, and ultimately destroy them by digestion. The phagocytes are attracted from the blood vessels and elsewhere towards the noxious See also:focus by the chemiotaxis exerted upon them by the toxins secreted by the bacteria contained within it. The chemiotaxis in this instance is positive, but the toxins from certain other bacteria may act negatively; and such bacteria are fraught with particular danger from the fact that they can spread through the body unopposed by the phagocytes, which may be looked upon as their natural enemies.

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