186 



PHYSIOLOGY 



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 paralyzed, 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 without 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 twitch 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 contains no nerve fibres (Fig. 42), and it is 

 only when a section containing nerve-fibres is exposed 

 to the action of glycerin that contraction takes 

 place. On the other hand, mere exposure of muscle 

 to the vapour of dilute ammonia causes con- 

 traction (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 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 

 FIG. 42. The ramification may }> e applied many times to any point on the 



of the nerve fibres within 



the sartorius muscle of muscle or nerve without killing the part stimulated, 



the frog showing the free- whereas w i tn other f orms o f stimulus it is difficult to 

 dom of the lower portion 



of the muscle from nerve 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 intensity and short duration 

 obtained from an induction coil. 



(1 ) CONSTANT CURRENT. As a source of constant current a Darnell'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, and if the binding screws of the two elements are connected by 

 a wire, the current flows in the wire (outer circuit) from copper to zinc, thus completing 

 the circuit. Since in the outer circuit the current flows from copper to zinc, the terminal 

 attached to the copper is called the positive pole, and that to the zinc the negative 

 pole. When the current is required to be very constant, the zinc may be immersed 

 in a saturated solution of zinc sulphate instead of dilute sulphuric acid. A Daniell's 

 cell, though very constant, gives only a small current, owing to its small electromotive 

 force and high internal resistance. 



When a stronger current is required it is best to use a storage battery. In this, 



