Page:The American Cyclopædia (1879) Volume I.djvu/552

520 ANIMAL ELECTRICITY the negative pole. According to this impor- tant law, when any point of the longitudinal section of a muscle is connected by a conduc- tor with any point of the transverse section, an electric current is established, which is di- rected in the muscle from the transverse to the longitudinal section. Du Bois-Reymond has discovered that the smallest part of a mus- cle acts in the same way as the whole of it, except that the strength of the current is less and less powerful as the part is smaller. Each elementary bundle 'of fibrils in a muscle seems to be like a couple in a galvanic battery, except that the couples represented by these elementary bundles are not able to transmit their current so freely as the couples of a real galvanic battery usually are. Du Bois-Rey- mond has found that the amount of electricity generated in muscles must be excessively great ; but as it is impossible to make an aggregation of all the elementary currents existing in a muscle, we have not a real measure of the quantity of electricity produced in these or- gans. We owe to Matteucci the discovery of one of the most important facts concerning animal electricity. He found that when a muscle contracts, if there is a nerve placed upon it leading to another muscle, the latter contracts also. The contraction of this second muscle Matteucci calls induced. To facilitate the understanding of what we have to say on this subject, we will call not only this second- ary contraction induced, but also the muscle that exhibits it, and we will call the first contrac- tion and the muscle in which it takes place in- ducing. Matteucci had a great deal of trouble in trying to explain this induced or secondary contraction; his latest view was that it re- sults from a galvanic discharge from the in- ducing muscle on the nerve of the induced one. Du Bois-Reymond, who has carefully examined the circumstances of this fact, explains it other- wise, lie supposes that the current of the in- ducing muscle passes through the nerve of the induced one, and that when the inducing mus- cle is set in contraction, the current dimin- ishes, and, as any diminution of a continuous current passing through a nerve is a cause of contraction for the muscle which it animates, it results that the induced muscle contracts. It is known that when a continuous current passes through a nerve there is a contraction in the muscle which it enters in the beginning of the passage and on its cessation, and also when there is any change in its strength. It is to this last condition that the induced contrac- tion is attributed by Du Bois-Reymond, but, if he were right, there should be a contraction in the induced muscle at the time we put its nerve on the inducing one, and also at the time we take it away; but unfortunately for the theory, there is no contraction in these cases, except in peculiar circumstances. We must therefore consider the theory of the distin- guished German physiologist as not sufficiently grounded. Whatever m%y be the cause of the irritation of the nerve of the induced mnscle, it is certain that when the inducing one con- tracts this motor nerve is irritated ; the same thing takes place, as Matteucci and Brown-Se- quard observe, when an exciter or a sensitive nerve instead of a motor is placed upon the inducing muscle; the irritation then causes either a reflex movement or a pain. Brown- Sequard has been led by many experiments to conclude that the irritation of sensitive nerves by the contraction of inducing muscles has a great share in many important physio- logical and pathological phenomena. Every one knows that, except when we look at the parts of our body which we move voluntarily, we direct our movements almost entirely ac- cording to the sensations that we receive from our contracting muscles. These sensations have been shown by this physiologist to be chiefly due to the induced irrritation of the sensitive nerves at the time the muscles con- tract. The muscular sense of Sir Charles Bell, or the guiding sensations of Prof. Carpenter, are thus obtained, and so it is with the measure of the distance of objects when looked at with both eyes ; the state of our ocular muscles teaches us the distance, and they do it by the irritation they induce in nerves while contract- ing. According to Brown-Sequard, the pain of cramps, that of the contractions of the uterus in parturition, that of the spasm of the sphinc- ters, &c., depends upon an excessive induced irritation of the sensitive nerves in conse- quence of muscular contractions. Among the other proofs adduced by him in support of his view that muscular contractions, normal or pathological, induce irritations in their sensi- tive nerve fibres, probably by a galvanic dis- charge, and exactly as an inducing muscle irri- tates a motor nerve placed upon it, the follow- ing are the most important: He has found that it is electrically just the same thing for the intensity of the irritation of the motor nerve lying upon an inducing muscle, and for the intensity of pain in a case of spasm of the sphincter of the anus, and in a case of contrac- tion of the anterior muscles of the thigh. In these three circumstances, viz., the experiment with the motor nerve, and the two pathologi- cal cases in man, we observe: 1, that there is no irritation or no pain if the inducing muscle has no resistance to overcome when it con- tracts (it is so after the section of the muscle or of its tendon) ; 2, that the irritation or the pain increases when the inducing muscle is ex- tended. The known facts that the pain due to the spasm of the sphincter of the anus disap- pears when it is divided, and that the section of a tendon of a contracted muscle causes the cessation of pain, had not hitherto received any explanation. The researches of Brown- Sequard render now very easy the understand- ing of the mode in which these facts are pro- duced. With the help of his very sensitive | galvanometer, Du Bois-Reymond has been able i to prove that the galvanic currents of muscles