644 



A MANUAL OF PHYSIOLOGY 



A nerve may be stimulated by an electrotonic current 

 produced in nerve-fibres lying in contact with it. A well- 

 known illustration of this is the experiment known as the 

 paradoxical contraction (Practical Exercises, p. 654). 



The current of action of a nerve can also stimulate 

 another nerve when the excitability of both is greater than 

 normal, as is the case in the nerves of frogs kept in the cold. 

 This comes under the head of secondary contraction. But 

 the best-known form of secondary contraction is where a 

 nerve, placed on a muscle so as to touch it in two points 

 (Fig. 212), is stimulated by the action-current of the muscle, 



and causes its own muscle to 

 contract. A secondary tetanus 

 can be obtained in this way by 

 dropping a nerve on an arti- 

 ficially tetanized muscle. The 

 beat of the heart causes usually 

 only a single secondary con- 

 traction when the sciatic nerve 

 of a frog is allowed to fall on it 

 (p. 179). But when the diphasic 

 variation is well marked, as it is 

 in an uninjured heart, there may 

 be a secondary contraction for 

 each phase, i.e., two for each 

 heart-beat. Excitation of one 

 muscle may in the same way 

 cause secondary contraction of 

 another with which it is in close contact. 



The electromotive phenomena of the heart and of the 

 central nervous system are naturally included under those 

 of muscle and nerve. 



Heart. In the frog's heart with each systole the diphasic variation 

 consists of a first phase, during which the base is negative to the 

 apex, and a second phase, during which the apex is negative to the 

 base. The meaning of this is that the negative electrical change, 

 like the contraction, starts at the base, and passes on to the apex. 

 Sometimes a third phase is seen (triphasic variation), in which the 

 base aga*o becomes negative to the apex. It has been supposed 

 that this is due to the contraction of the arterial bulb, which follows 



FIG. 212. SECONDARY CON- 

 TRACTION. 



The nerve of muscle M touches 

 muscle M' at a and b. Stimulation 

 of the nerve of M' at S causes con- 

 traction of M. 



