iv GENERAL PHYSIOLOGY OF NERVOUS SYSTEM 225 



propagation through the nerve, owing to the resistance encountered. 

 Ducceschi in fact saw that on compressing the frog's sciatic lightly 

 by his method (Chap. IV. p. 193) near the muscle, and then 

 tetanisiug it with an induced current alternately near the point 

 of compression and at the central end of the nerve, the conduction 

 of the impulse to the muscle ceased earlier from the more distant 

 points of excitation than from those nearer to the muscle. 



Whatever the degree of excitability in the different nerves, it 

 can survive for a long time, independently of the circulation. If 

 care be taken to avoid desiccation and too sudden changes of 

 the normal temperature, the medullated fibres of mammalian 

 nerves are capable of preserving their excitability for many hours, 

 and those of the frog for many days, even when the circulation 

 has been entirely arrested. 



The functions of nerves are usually supposed to be very 

 unstable and readily altered by slight causes. But it is easy to 

 demonstrate, on the contrary, that nerve, owing to its low meta- 

 bolism and specific differentiation, represents a form of living proto- 

 plasm which is endowed with peculiarly high resistance to noxious 

 influences. It is possible to experiment for a long time with a 

 mammalian nerve, after it has been isolated for a considerable 

 distance from the surrounding tissues and its circulation cut off, 

 without loss of its normal functions, provided it remains covered 

 and protected from heat and cold, and that circulation is normal 

 in the central and peripheral organs with which it is connected. 

 After occluding the aorta of a rabbit, the sciatic (according to 

 Fredericq) is capable, on electrical stimulation, of causing the 

 corresponding muscles of the leg to contract, even after an interval 

 of half an hour. After three-quarters of an hour the contractions 

 cease for indirect stimulation, while the direct excitability of the 

 muscles still persists. This is due not to exhaustion of the nerve, 

 but to loss of conductivity in the motor end-plates. In fact even 

 when the muscles have lost their excitability the nerves are still 

 alive and capable of excitability and conductivity, as is shown by 

 the negative electrical variation. 



The most striking demonstration of the vital resistance of 

 nerve is, however, its comparative non-fatigability. 



When a motor nerve is excited, the muscle apparently becomes 

 fatigued long before the nerve. This was demonstrated by 

 Bernstein in the following experiment : Make two preparations of 

 the frog's sciatic ; cut them high up so as to separate them from 

 the spinal cord, to exclude sensations and reflexes : tetanise the 

 two peripheral stumps simultaneously with the same induced 

 current, and at the same time pass a strong constant current in 

 the ascending or descending direction through one of the two 

 sciatics below the point tetanised : this by a polarising process 

 known as electrotonus, which we shall presently study inhibits the 



VOL. Ill Q 



