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that the currents undergo a diminution or negative variation, a change which passes along the nerve in the form of a wave and with a velocity equal to the rate of transmission of the nerve impulse. The wave length of a single negative variation has been estimated to be 18 millimeters; the period of its duration being from 0.0005 to 0.0008 of a second.

It is asserted by Hermann that perfectly fresh, uninjured muscles and nerves are devoid of currents, and that the currents observed are the result of a molecular death at the point of section, this point becoming negative to the equatorial point. He applies the term “ action currents” to the currents obtained when a muscle is thrown into a state of activity.

Electrical Properties of Nerves.— When a galvanic current is made to flow along a motor nerve from the center to the periphery, from the positive to the negative poles, it is known as the direct, descending, or centrifugal current. When it is made to flow in the reverse direction, it is known as the inverse, ascending, or centripetal current.

The passage of a direct current enfeebles the excitability of a nerve; the passage of the inverse current increases it. The excitability of a nerve may be exhausted by the repeated applications of electricity; when thus exhausted it may be restored by repose, or by the passage of the inverse current if the nerve has been exhausted by the direct current, or vice versa.

During the actual passage of a feeble constant current in either direction neither pain nor muscular contraction is ordinarily manisested; if the current be very intense, the nerve may be disorganized and its excitability destroyed.

Electrotonus.—The passage of a direct galvanic current through a portion of a nerve excites in the parts beyond the electrodes a condition of electric tension or electrotonus, during which the excitability of the nerve is decreased near the anode or positive pole, and increased near the kathode or negative pole; the increase of excitability in the katelectrotonic area, that nearest the muscle, being manifested by a more marked contraction of the muscle than the normal, when the nerve is irritated in this region. The passage of an inverse galvanic current excites the same condition of electrotonus; and the diminution of excitability near the anode, the anelectrotonic area, that now nearest the muscle, being manifested by a less marked contraction than the normal when the nerve is stimulated in this region. Between the electrodes is a neutral point where the katelectrotonic area emerges into the anelectrotonic area. If the current be a strong one, the neutral point approaches the kathode; if weak, it approaches the anode.

When a nervous impulse passes along a nerve, the only appreciable effect is a change in its electrical condition, there being no change in its tempera

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ture, chemical composition, or physical condition. The natural nerve currents, which are always present in a living nerve as a result of its nutritive activity, in great part disappear during the passage of an impulse, undergoing a negative variation.

Law of Contraction.-If a feeble galvanic current be applied to a recent and excitable nerve, contraction is produced in the muscles only upon the making of the circuit with both the direct and inverse currents.

If the current be moderate in intensity, the contraction is produced in the muscle both upon the making and breaking of the circuit, with both the direct and inverse currents.

If the current be intense, contraction is produced only when the circuit is made with the direct current, and only when it is broken with the inverse current.

The Reaction of Degeneration.—Two different applications of electricity are used in electro-physiology and electro-therapeutics—the constant or galvanic, and the interrupted or faradic currents. Injured and paralyzed muscles and nerves react differently to these two kinds of stimuli, and the facts are of the greatest importance in the diagnosis and therapeutics of the precedent lesions. The principal difference of behavior relates to the reaction of degenerationa condition produced by paralysis of any kind. It is characterized by a diminished or abolished excitability of the muscles to the faradic current, while there is at the same time an increased excitability to the galvanic current. The synchronous diminished excitability of the nerves is the same for either current. The term partial reaction of degeneration is used when there is a normal reaction of the nerves, but the muscles show the degenerative reaction. This condition is a characteristic of progressive muscular atrophy.

CRANIAL NERVES. The Cranial Nerves come off from the base of the brain, pass through the foramina in the walls of the cranium, and are distributed to the skin, muscles, and organs of sense in the face and head.

According to the classification of Sæmmering, there are 12 pairs of nerves, enumerating them from before backward, as follows, viz:ist Pair, or Olfactory.

4th Pair, or Patheticus, Trochlearis. 2d Pair, or Optic.

51h Pair, or Trifacial, Trigeminus. 3d Pair, or Motor oculi communis. 6th Pair, or Abducens..

7th Pair, or Facial, Portio dura. Ioth Pair, or Pneumogastric. 8th Pair, or Auditory, Portio mollis. Iith Pair, or Spinal accessory. 9th Pair, or Glosso-pharyngeal. 12th Pair, or Hypoglossal.

The Cranial Nerves may also be classified physiologically, according to their function, into three groups : I. Nerves of special sense. 2. Nerves of motion. 3. Nerves of general sensibility.

Ist Pair. Olfactory. Apparent Origin.-From the inferior and internal portion of the ante. rior lobes of the cerebrum by three roots, viz: an external white root, which passes across the fissure of Sylvius to the middle lobe of the cerebrum ; an internal white root, from the most posterior part of the anterior lobe; a gray root, from the gray matter in the posterior and inner portion of the inferior surface of the anterior lobe.

Deep Origin.—Not satisfactorily determined.

Distribution.—The olfactory nerve, formed by the union of the three roots, passes forward along the under surface of the anterior lobe to the ethmoid bone, where it expands into the olfactory bulb. This bulb contains ganglionic cells, is grayish in color and soft in consistence ; it gives off from its under surface from fifteen to twenty nerve filaments, the true olfactory nerves, which pass through the cribriform plate of the ethmoid bone, and are distributed to the Schneiderian mucous membrane. This membrane extends from the cribriform plate of the ethnoid bone downward, about one inch.

Properties.-—The olfactory nerves give rise to neither motor nor sensory phenomena when stimulated. They carry simply the special impressions of odorous substances. Destruction or injury of the olfactory bulbs is attended by a loss of the sense of smell.

Function.—Governs the sense of smell. Conducts the impressions which give rise to odorous sensations.

2d Pair. Optic. Apparent Origin.-From the anterior portion of the optic commissure.

Deep Origin.—The origins and connections of the optic tract are very complex. The immediate origins are bands of fibers from the thalamus opticus and anterior corpora quadrigemina. The corpora geniculata are interposed ganglia. The ultimate roots are traced1. By a broad band of fibers—“the optic radiation of Gratiolet”--to the

psycho-optic centers in the occipital lobes.

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