254 CORRELATION OF NERVOUS FORCE AND ELECTRICITY.

action, as the two metals of the galvanic battery, or the grey and white substances of nervous tissue.

Electric conductors require to be insulated by some non-conductor, as gutta-percha, &c.; and the white substance of nerve-tubes is regarded as analogous. Electricity requires a circle to be constituted between the opposite plates; and nervous force seems to travel in the same way by the looped arrangement of the nerves. Electricity excites motion if applied to motor nerves, and subjective sensations in sensitive ones; but so do many mechanical and chemical stimuli. The effect on a motor or sensitive nerve ceases the moment the electric current is stopped-not so if a ganglion be acted on, as then the influence seems stored up, and continues for some time after the electric conductors are removed. The phenomena of electric fishes, which will be alluded to on a future page, are thought to support the identity of those two agents. Arguments against the identity of nervous force and electricity are abundant-thus, nervous matter does not act on the ordinary tests for electricity, as the magnetic needle, the galvanometer, electrochemical decomposition, the electric spark; again, a tight ligature prevents the conduction of nervous force, not of an electric current. If a nerve be divided and the extremities united by good conductors, as wire or moist thread, electricity, but not nervous force, is carried on. Nervous tissue is a very bad conductor of electricity, even worse than some other tissues, being but one-fourth as powerful as muscle. When the arguments on either side are carefully weighed, they at least warrant the conclusion that electricity and nervous force are correlated.

The Theory of Polarity was first applied to nervous force by Todd and Bowman. It supposes some molecular change occurs, by which a different arrangement of particles is effected in nervous tissue. If the quiescent state be represented by the letters AB AB AB, the particles during action may be supposed to arrange themselves as A BA BA B, which is termed the polar state. Recent ex

periments show that the human body usually indicates free electricity, and that its kind varies with sex- -that of the male being positive, of females negative. The skin and mucous surface are in these opposite states, and no parts of the body, save those that correspond exactly on different sides, are quite equal in the degree of electrical disturbance. Alcohol is said to increase, and rheumatism to diminish almost to zero, this condition. In the frog and other batrachians there is a "courant propre," for the disturbance runs from the hind feet towards the head in the direction of the strongest muscular current; while in man's arm at rest Du Bois Reymond finds it runs from shoulder to fingers. He finds that a scratch on one hand destroys its equilibrium to the other. Matteucci has performed many valuable experiments, and uses as the most sensitive galvanoscope a prepared frog. He asserts. that electric current is increased during the contraction of a muscle. It is found there is also a current through nerves, which, strange to say, runs in the same

direction in both motor and sensitive Matteucci's Pile. nerves. Matteucci's pile is constructed by placing sections of the thighs of newly-killed frogs in the manner here shown, when it is found that a current of the negative kind passes transversely, and a positive current longitudinally.

There are many instances of the abundant production of electricity in some individuals—thus, a lady is stated, in the "American Journal of Medical Science" (January, 1838), to have given 4 sparks per minute to a brass ball, 1 inches in distant, when insulated by a carpet. This condition suddenly appeared during the occurrence of an aurora borealis, and gradually disappeared after some months. A temperature of 80° and high spirits promoted the electrical state, and cold or depressing emotions checked it. The influence of electricity on the

human body has been much exaggerated; it has been even considered capable of accounting for that physical impossibility" spontaneous combustion."

The Electric Fishes are the torpedo, gymnotus, trichiurus, malapterurus, and tetraodon. The torpedo grows to 4 feet in length, and has weighed 73 lb. The French peasants use all parts, except the electric organs, as food. Symmetrically beside the head, as shown in

Electrical Organs of the Torpedo, supplied by the pneumogastric nerves. the annexed figure are two great masses of nervous matter arranged in columns, and connected with the tri

facial and vagus nerves. These are the electric organs, and the resemblance between them and many voltaic arrangements is striking. Dr. M'Donnell, of the Car michael School, has discovered organs homologous to them in the common skate. The gymnotus may attain a length of 20 feet. Its organs are arranged in laminæ, run the whole length, and form the chief bulk of the animal. The electricity developed by these creatures is identical with that generated by the galvanic battery, the frictional machine, and other means, as the following facts will show: 1. Shocks have been felt by 27 persons who joined their hands, the first and last touching the fish. 2. The galvanometer needle has been deflected to 33°. 3. A magnet has been made. 4. Chemical de

composition or electrolysis of iodide of potassium and other substances has been effected. 5. Heat has been evolved, even sufficient to fuse gold leaves. 6. The spark has been obtained. Spallanzani felt the spark from the foetus just removed from a pregnant torpedo. All electric power ceases the instant the animal dies, or when the connexion between the organs and the nervous system is severed, but by irritating the end of the nerve connected with the organ, discharges are produced. It seems controlled by the will of the animal, and after repeated discharges, it loses the power, and is then easily captured. While thus convinced of the identity of animal with other kinds of electricity, Schönbein remarks: "So long as we are without an exact idea of what electricity is, the different modes of its development will of course be incomprehensible to us, even though anatomists and physiologists should have very carefully studied the structure of the fish ;" and we must conclude that the forces are not identical, though correlated, as the physical forces have been shown to be by Groves. Growing plants give rise to electric actions, which shows that the presence of a nervous system is not essential.

Luminosity, which some invertebrates, and occasion

ally even the human body evolves, has been said to be due to electricity. The phosphorescence observed in the tracks of vessels is due to minute infusoria and medusæ, the nervous system of which, according to Ehrenberg, produces the phenomenon; but in some luminous animals, noctiluca, for instance, no nervous system can be demonstrated. Among insects, the fire-fly and glow-worm emit light, particularly about the reproductive organs. It continues very slightly after death, but may be increased, as Darwin found, by mechanical irritation. It is due to oxidation, for if the creatures are placed in any irrespirable gas, or in vacuo, it ceases. Phosphorus is probably the material which is slowly oxidised, for we can produce great luminosity by injecting an oily solution of that substance into the veins of a dog. The same appearance has been remarked in the urine of persons to whom that drug was administered; and in cases of poisoning by phosphorus, the luminosity of the viscera is the main test for its diagnosis. Sir H. Marsh-to whom we owe almost all our information on the evolution of light from the human body-records that the individuals in whom he noticed it were suffering from phthisis, and death was approaching a degree of decomposition, however, preceding it. The breath had a peculiar putrid odour. In one case the light appeared 10 days before death. It was "silvery, like the reflection of moonlight on water, or like that of the aurora borealis."

In one case, Dr. Stokes found so much light emitted from a cancerous ulcer, that the hands of a watch-dial could be distinguished in the dark. Bodies in the dissecting room, and many decomposing animal substances, often show a phosphorescence, which can be communicated to any object coming in contact with them.

We shall now proceed to examine in detail the divisions of the cerebro-spinal centre:

Three Membranes clothe the brain and spinal cord: 1. The dura mater, a white fibrous covering which sup