TRANSMISSION OF SENSORY AND MOTOR IMPRESSIONS. 579 



a given time ; and on the contrary, whatever may be the nature of the stimu- 

 lus applied, whether mechanical, chemical, thermic, or electrical, neither 

 sensation in a sensory nerve nor motion in a motor nerve is produced when 

 the passage from one grade of intensity to another is extremely gentle. If 

 for example mechanical pressure be so applied to a motor nerve as that com- 

 mencing with the slightest possible contact it may steadily and continuously 

 increase in force even till the nerve be killed at that spot, no convulsions 

 occur in the muscles to which the nerve is distributed. 1 Muscular contrac- 

 tion only supervenes where the change in the amount of pressure, whether 

 greater or less than that previously present, is so sudden as to produce a kind 

 of shock. In proportion to the energy of the shock or vibration of the par- 

 ticles of the nerve is the effect produced on the muscles; thus we see that 

 tetanic convulsions may be induced by violent extension of a nerve, and 

 that the whole body of a decapitated Frog can sometimes be rendered rigid 

 by throwing it on the ground. In a similar manner, whilst the sudden ap- 

 plication of a ligature or of heat to a zenxory nerve produces acute pain, 

 numbness passing into entire insensibility is the only effect if they be grad- 

 ually applied, 2 and the same holds good for all other kinds of stimuli. In 

 all instances the passage of a nerve from a state of rest to one of functional 

 activity is accompanied by changes in the chemical constitution of the tissue, 

 and it has been shown that whilst the substance of the brain, spinal cord, 

 and nerves is feebly alkaline or neutral when at rest, it becomes distinctly 

 acid after long and exhausting activity. 3 The molecular and chemical 

 changes seem to be more readily induced in sensory than in motor nerves, 

 the latter requiring more powerful stimulation to call them into play than 

 the former, their equilibrium in other words being more stable. Chauveau 4 

 has endeavored to explain the effects of electrical currents on the supposition 

 that they produce a mechanical commotion in the particles of the nerve. 

 Hence induced currents, which are currents of high tension, and therefore 

 produce a more powerful shock or series of shocks, constitute stronger stimuli 

 than continuous currents, in which the quantity of electricity is large. 5 On 

 applying a continuous current to a nerve contraction takes place in the 

 muscle it supplies, and a similar contraction occurs at the moment when the 

 current is broken, because at these periods, in accordance with Du Bois-Rey- 

 mond's law, there is a sudden change in the molecular constitution of the 

 nerve. When the interruptions of a continuous current are about twenty in 

 the second, the muscle has not time to relax between the periods of breaking 

 and making contact and passes into a state of tetanus. The induced cur- 

 rents which are developed in a secondary coil on closing and opening the 

 current in a primary coil, as in the instruments of ordinary construction, are 

 always of short duration, and the molecular movement produced in the 

 nerve is correspondingly rapid and sudden. The duration of the opening 

 current does not exceed 0.0042", whilst the duration of the closing current 

 is 0.0114". The closing current is comparatively weak, its proportion to 

 the opening current being as 1 : 6, the latter being of higher tension and of 

 shorter duration. Chemical stimuli act but slowly on nerves, which we may 

 attribute to the sheath of the fibres affording a certain amount of protection 



1 Schiff, Physiologic, 1839, p. 94. 



2 See Heinzmann. Ueber die Wirkung eehr allmabliger Aenderungen thermische 

 Keize atif die Empfindungsnerven, Pfliigcr's Archiv, Bd. vi, p. 2'22. 



3 By Funke, Gscheidh-n, and Ranke, Centrnlblatt, 1869, p. 97. 



4 Chanveau, Brown-Sequard's Jmirn. de la Physiologic, 1859, p. 576. 



5 For an account of the action of induced and continuous currents on healthy and 

 on paralyzed nerve and muscle respectively, see Onimus, Kobin's Journal de 1'Anat- 

 omie, 1874, translated in full in the Practitioner, 1874-75. 



