EXCITATION OF MUSCLE 



207 



so we must conclude that the curare paralyses the muscles by affecting 

 the terminations of the nerve within the muscle, and probably the 

 end-plates themselves. This experiment teaches us that muscle can 

 be excited to contract by direct stimulation, even when the terminal 

 ramifications of the nerve within it are paralysed, so that stimulation 

 of them would be without effect. 



The same fact may be demonstrated in a different way by means 

 of chemical stimuli. It is found that whereas 

 strong glycerin excites nerve fibres, it is with- 

 out effect on muscle fibres , while on the other 

 hand weak ammonia is a strong excitant for 

 muscle, but is without effect on nerve. If the 

 frog's sartorius be dissected out and the lower 

 end dipped in glycerin, no effect is produced. 

 On snipping off the lower third of the muscle 

 and then immersing the cut end in glycerin, 

 a twitch at once occurs. The lower end con- 

 tains no nerve fibres (Fig. 42), and it is only 

 when a section containing nerve-fibres is ex- 

 posed to the action of glycerin that contraction 



takes place. On the other hand, mere exposure FIG. 42. The ramification 

 of muscle to the vapour of dilute ammonia 

 causes contraction (and subsequent death), 

 although the nerve to the muscle can be 

 immersed in the solution without any excitation 

 being produced. 



Of all the different stimuli that we have mentioned as capable of 

 exciting muscular contraction, the electrical is that most frequently 

 employed. It is easy, using this form, to graduate accurately the 

 intensity and duration of the stimulus. At the same time the stimulus 

 may be applied many times to any point on the muscle or nerve with- 

 out killing the part stimulated, whereas with other forms of stimulus 

 it is difficult to obtain excitatory effects without injuring to a greater 

 or less extent the part stimulated. 



METHODS EMPLOYED FOR THE STIMULATION OF MUSCLE AND 

 NERVE. The two commonest forms of electrical stimuli employed are (1) the 

 make and break of a constant current, (2) the induction currents of high inten- 

 sity and short duration obtained from an induction coil. 



(1) CONSTANT CURRENT. As a source of constant current a Daniell's cell 

 is generally employed. This consists of an outer pot containing a saturated 

 solution of copper sulphate, in which is immersed a copper cylinder. To the 

 cylinder at the top a binding screw is attached, by which the connection of the 

 copper with a wire terminal is effected. Within the copper cylinder is a second 

 pot of porous clay, filled with dilute sulphuric acid, in which is immersed a rod 

 of amalgamated zinc. In this cell the zinc is the positive and the copper the 

 negative element. Hence the current flows (in the cell) from zinc to copper, 



nerve fibres 

 the sartorius 

 muscle of the frog, 

 showing the freedom 

 of the lower portion 

 of the muscle from 

 nerve fibres. (KiJHNE.) 



