68 A SIMPLE MUSCULAR CONTRACTION. [BOOK i. 



a moderately thick wire w, offering a certain amount only of resistance, 

 is interposed between the upper binding screw a' on the pillar a, and 

 the binding screw c leading to the primary coil. Under these arrange- 

 ments the current from the battery passes through a', along the inter- 

 posed wire to c', through the primary coil and thus as before to m. 

 As before, by the magnetisation of m, e is drawn down and b brought 

 in contact withy*. As the result of this contact, the current from the 

 battery can now pass by a, f, and d (shewn by the thin interrupted line) 

 back to the battery ; but not the whole of the current, some of it can 

 still pass along the wire w to the primary coil, the relative amount 

 being determined by the relative resistances offered by the two courses. 

 Hence at each successive magnetisation of m, the current in the 

 primary coil does not entirely disappear when b is brought in contact 

 with f\ it is only so far diminished that m ceases to attract e, and 

 hence by the release of b from f the whole current once more passes 

 along w. Since, at what corresponds to the ' break ' the current in 

 the primary coil is diminished only, not absolutely done away with, 

 self-induction makes its appearance at the 'break' as well as at the 

 'make'; thus the 'breaking' and 'making' induced currents or shocks 

 in the secondary coil are equalized. They are both reduced to the 

 lower efficiency of the 'making' shock in the old arrangement; 

 hence to produce the same strength of stimulus with this arrange- 

 ment a stronger current must be applied or the secondary coil pushed 

 over the primary coil to a greater extent than with the other arrange- 

 ment. 



The Phenomena of a simple Muscular Contraction. 



45. If the far end of the nerve of a muscle-nerve preparation, 

 Figs. 1 and 3, be laid on electrodes connected with the secondary 

 coil of an induction-machine, the passage of a single induction- 

 shock, which may be taken as a convenient form of an almost momen- 

 tary stimulus, will produce no visible change in the nerve, but the 

 muscle will give a twitch, a short sharp contraction, i.e. will for an 

 instant shorten itself, becoming thicker the while, and then return 

 to its previous condition. If one end of the muscle be attached to 

 a lever, while the other is fixed, the lever will by its movements 

 indicate the extent and duration of the shortening. If the point 

 of the lever be brought to bear on some rapidly travelling surface, 

 on which it leaves a mark (being for this purpose armed with a 

 pen and ink if the surface be plain paper, or with a bristle or 

 finely pointed piece of platinum foil if the surface be smoked glass 

 or paper), so long as the muscle remains at rest the lever will 

 describe an even line, which we may call the base line. If how- 

 ever the muscle shortens the lever will rise above the base line 

 and thus describe some sort of curve above the base line. Now 



