Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/38

Rh PHYSIOLOGY [NERVOUS bellura flask -shaped, having processes at each end. A nerve-cell shows a large clear nucleus and a small nucleolus, whilst the cell- substance is very granular. Some observers think they have traced into the siibstance of the cell a fibrillated structure from the axis- cylinder of the nerve-fibre ending in the pole or process ; but this is doubtful, and the appearance may be accounted for by the action of the reagent employed and by the great difficulty of correctly interpreting optical apj&amp;gt;earanees under very high powers. The neuroglia is a delicate interstitial connective substance having small connective-tissue corpuscles imbedded in it. Chemical Chtmical Constitution of Grey and Jfhite Matter. This is still sub- imperfectly known, and throws almost no light on the functions of stances the central organs. By various chemical processes the following substances have been obtained from nervous matter : cerebrin, lecithin, albumin, neurokeratin, cholesterin and fats, creatin, xanthin, hypoxaiithin, inosite, lactic acid, volatile fatty acids, salts, and water. The grey matter of the brain is distinguished chemically from the white chiefly by containing more water, albumin, lecithin, and lactic acid, and less cholesterin, fat, and protagon (Hermann). Doubtless many of these substances are de rived from the disintegration of a more complex chemical substance not yet isolated in a pure state from nervous matter. Petrowsky gives the composition of grey and white matter as follows. Grey matter. White matter. Water 81 -G 68-4 Solids. 1S 4 31 G The solids consist of 55-4 27-7 17 2 9 9 18 7 51 9 5 9 5 Substances soluble in ether 67 3-3 Salts 1 5 (3 The salts found in nervous matter are similar to those in blood, and it would appear that phosphates, or rather combinations in which phosphorus exists, are the most prominent products of analysis. Thus about 40 per cent, of the salts consist of phos phates of soda and of potash that is, the ash, on analysis, gives this result ; but it must not be inferred that in nervous tissue phosphates of the alkalis exist to this amount, as there is every reason to think that phosphorous compounds, along with alkalis, exist in nervous matter, although not in the form usually called phosphates. The remarkably large amount of water, amounting to no less than from 70 to 80 per cent., indicates matter in a condition suitable for rapid molecular changes, on which, no doubt, the functions of the tissue depend. Excitability of Grey Matter. As grey matter contains both nerve- fibres and nerve-cells, and as these cannot be separated in any experi ment, it is clear that no precise results can be obtained from any effort to distinguish the excitability of grey matter from that of white. The excitability of the grey matter must depend on blood- supply and on the rapid removal of waste -products. If the first be deficient either in quantity or quality, or if the second be not carried on so rapidly as to get rid of the waste-products as they are formed, the activity of the nerve-cells must suffer. The sudden deprivation of blood, as when the heart ceases to beat for even half a second, will cause unconsciousness ; the mixture with the blood of a small quantity of bromide of potassium, or of alcohol, or of chloroform or other anaesthetic, or of morphia, will affect the activity of the brain. And it is well known that, when disease of the kidney, or such a disease as an acute fever, affects the body, matters may accumulate in the blood which so contaminate it as to make it unfit to carry on the vital changes on which activity of brain depends, and the result is delirium or unconsciousness. There is every reason to believe that the activity of nerve-cells is delicately attuned to surrounding conditions. A small excess per cent, of carbonic acid, or a small amount of what we call a poison, is sufficient to modify or arrest their action. The rhythmic action of various centres, such as those controlling the movements of re spiration, is in favour of the view that the activity of such centres depends on delicate equipoises. If during expiration there is for the moment a deficiency of oxygen in the blood, or an accumula tion of carbonic acid, the result will be an attempt at inspiration. This gets rid of the carbonic acid and introduces oxygen, and an expiration ensues. It is not pretended here to state what exactly happens, as these phenomena of respiration are still obscure, but they are brought forward with the view of showing that the actions of the rhythmic centres of respiration depend on the delicate balance established between the external conditions and those centres. If this be the case there is little doubt that a similar effect is produced on other centres by the nature of the blood supplied, and that the quality and quantity of the supply are important factors in the production of all conscious conditions. General Phenomena manifested by Nervous Centres. Before enter ing on a detailed description of the functions of the great centres such as spinal-cord and brain, it is well to take a survey of some of the general phenomena manifested by such centres. These may be grouped under the heads of (1) reflex actions, (2) inhibitory actions, (3) accelerating actions, (4) vaso-motor actions, (5) secretory actions, (6) sensations, and (7) intellectual acts. Reflex Actions. Impressions made on sensory nerves are con- Refit x veyed to nerve-centres, where they may or may not awaken con- actions, sciousness. A sensation may be defined as the consciousness of an impression, and may or may not be followed by motion. Either motion may be voluntary, or it may be caused by direct stimulation of the motor nerve distributed to the muscles. The latter kind of action in the living body is not common. Usually motor nerves are acted on by the will or by emotional states ; but it not unfrequently happen! that physical stimuli occasion motion in an indirect manner, the impressions being carried along sensory nerves to a central organ, where changes are excited which result in a discharge of nervous energy along motor nerves to various muscles. Thus a frog in which the brain and inedulla oblongata have been destroyed will draw up its limbs if the foot be pinched. Such actions, taking place without consciousness, are called &quot;reflex actions,&quot; and the mechanism required for their performance may be thus described: (1) excitation of a sensory or afferent nerve, (2) excitation of an intermediate I nervous or reflex centre, and | (3) excitation of a motor or efferent nerve, which causes a | muscular contraction. The dia gram in fig. 6 shows the sim plest mechanism ; but it is rare | to find the arrangements simple, and the mechanism may become more complex (see fig. 7) either by the existence of a number of cells or groups of cells in the nerve-centre, or by the existence of numerous afferent or efferent nerves. The essence of a reflex action is the transmutation by means of the Fui. (i. Simple reflex action ; 1, sensory surface ; 2, muscle ; c, sensory nerve ; b, nerve -cell ; c, motor nerve. Thu arrows indicate the direction in which the influence travels. FIG. 7. Double reflex action, or action in which two or more nerve-cells are involved ; 1, 2, as in fig. (&amp;gt; ; a, motor nerve ; b, c, nerve cells. irritable protoplasm of a nerve-cell of afferent into efferent impulses (Foster). The following is a brief summary of the leading facts relating to reflex action. (a) The initial excitation may occur both in nerves of general sensibility and in those of the special senses ; but certain nerves more easily excite reflex actions than others. Thus when light falls on the retina there is contraction of the pupil, the afferent nerve in this case being the optic (see vol. viii. p. 821 sq.). (b) A reflex movement may occur whether we excite a sensory nerve at its commencement or at some point in its course, but in the latter case the action is less intense than in the former. (c) Grey matter containing nerve-cells constitutes the chief por tion of reflex centres, and groups of such reflex centres are frequently associated by internuncial fibres. The excitability is increased when these centres are severed from communication with psychical centres which preside over voluntary movements. Thus, after decapitation, reflex movements occur with greater intensity than in the injured animal ; they are also more active during sleep. It is evident, therefore, that reflex I actions may be restrained or hindered I in their development by the action of I higher centres. This is termed the f &quot;inhibition of reflex action.&quot; (d) Reflex movements may occur in I one muscle, or in many muscles or | groups of muscles. One or more groups of muscles may be involved I according to the strength of the stimulus applied to the sensory sur- 1 face and the degree of excitability of | the reflex centre at the time (see fig. 8). The facts are thus summarized by Pfliiger. Unilateral action : if in a decapitated frog we excite the skin of Fro. 8 p, sensory surface; a, 7&amp;gt;, the hind foot p, the excitation is c - (/ - e - nerve-oalla ; l, 2, 3, 4, 5, transmitted from the centre a to the y&amp;gt; 5&amp;gt; &quot; n muscles 1 of the foot on the same side. Symmetrical action ; if the excitation be more intense, it is transmitted to a centre on the