THE NERVOUS SYSTEM 431 



of electrotonic currents is not fully agreed upon ; they are markedly 

 shown in fresh medullated nerves, but not in those which are dead ; 

 they are only feebly shown in non-medullated nerves. Though 

 physical in origin, they can only be obtained in nerves which still 

 possess the characteristics of living. They may be demonstrated 

 in an artificial nerve in which the axis cylinder is represented by 

 platinum wires, and the medulla by a glass tube containing a solution 

 of zinc sulphate. It is believed that electrotonic currents are due 

 to polarisation occurring between the medullary sheath and axis 

 cylinder, but nothing is yet settled. 



The facts which have been ascertained experimentally as to effects 

 which follow the application of constant and induced currents have 

 been employed in the diagnosis of nerve degeneration. A healthy 

 motor or sensory nerve, or healthy muscle, reacts to both stimuli. 

 A degenerated nerve conveys no sensory impulses, nor is stimulation 

 of a motor nerve so affected followed by muscular contraction. 

 Muscles in the horse, other than as the effect of azoturia, do not 

 appear to suffer from atrophy, excepting as the result of injury or 

 degeneration of the nerves. A muscle will grow smaller if thrown 

 out of use, but the result is very different from the atrophy occurring 

 in consequence of nerve degeneration, which is remarkable for its 

 completeness and the relative rapidity with which it occurs. Muscle 

 fibres will respond to direct stimulation of a constant current, even 

 when the nerve is degenerated ; but the contraction is slower, and is 

 not brought about through the nerves, but through the muscle 

 fibres. A healthy muscle responds as readily to a constant as to 

 an induced current. With complete degeneration of a motor nerve, 

 no response on stimulation is evoked from the muscle ; nor can the 

 latter be stimulated by induction shocks, but it may be stimulated by 

 the constant current. This is employed in human practice as a test 

 of degeneration. A normal contraction, we have seen, occurs at 

 the negative pole on closing. To obtain a contraction at the positive 

 pole on closing, a stronger current must be employed ; but in para- 

 lysis, which is not of central origin, the muscles do not respond to 

 an induced current, but react to a continuous current, and the same 

 strength of current, both of opening and closing, will readily produce 

 a response. This is the Reaction of Degeneration. 



Negative Variation. — We have seen (p. 428) that when a nerve is 

 indicating through the galvanometer the existence of a current of 

 injury, its stimulation leads to a current in the opposite direction, 

 or negative variation. In living nerves negative variation has been 

 observed in consequence of stimulation of the brain, and the associa- 

 tion of negative variation with normal nerve impulses is accepted. 

 Negative variation marks in a nerve what a contraction represents 

 in a muscle — viz., the passage of an impulse. The capillary electro- 

 nometer shows that the onward rush of negativity travels at the 

 same rate as a nerve impulse. By means of the string galvanometer 

 of Einthoven (an instrument far surpassing the capillary electrono- 

 meter in delicacy), a negative variation may be seen to occur in 

 the vagus at each inspiration. Movements due to an electrical change 

 can be detected passing both up and down the nerve in consequence 

 of nervous impulses. 



Diphasic Variation. — When a muscle contracts, a wave runs along 

 it, and an electrical change occurs. The contracting part of the 

 muscle becomes more positive, and then, as the muscle returns to the 

 state of rest, this positivity disappears (see p. 407). This two-phase 



