CHAP, rv'.] PROPERTIES OF NERVOUS FIBRES. 395 



exciting the cut nerve, we must in fact bring excitation to bear 

 upon a point nearer and nearer the terminal extremities, till we 

 reach the muscles themselves. This gradual abolition of mo- 

 tricity corresponds > in this case, to an appreciable alteration in 

 the constitution of the nerve, that is to say, a progressive 

 coagulation of the medullary substance, the myeline of the 

 r nervous fibre. But, while the peripheric tronon of the nerve 

 thus dies little by little, the central end, which is always in 

 continuity with the central cell, lives intact, preserving all its 

 properties. 



How is this curious influence of the central cell upon the fibre 

 exercised ? What is, essentially, the character of the molecular 

 shocks transmitted by the central cell to the motory fibre ? 

 These are questions which still wait for an answer. Though 

 electricity is produced in the nervous tissue as in every living 

 tissue, it is not likely that the molecular shock which runs along 

 the motory fibres is of an electric nature. In effect, a simple 

 ligature, placed upon its course, is sufficient to stop it completely, 

 by disorganising the nervous fibre at a single point, whilst it 

 opposes no obstacle to the passage of an electric current. 

 Finally, the rate of propagation of the motory excitation along 

 a nerve is extremely slow. In fact it is proved by the experi- 

 ments of Helmholtz, Dubois-Reymond, &c., that in the motory 

 nerve of a warm-blooded animal this rate is only from 50 to 60 

 metres in a second. In a nerve cooled to zero it diminishes by 

 nine -tenths, and we can even arrest all conductibility in the 

 sciatic nerve of a frog by applying ice to one point of this 

 nervous trunk. 1 After that, it is very natural to see the 

 speed of nervous transmission retarded in cold-blooded animals, 

 whose temperature is at once lower and more variable than 

 that of warm-blooded animals. In effect, in the frog, for 

 example, this speed does not exceed from 15 to 20 metres in a 

 second. 



1 E. Onimus, De la Thtorie Dynamique de la CJialeur dans les Sciences 

 Biologiques. Paris, 1866, p. 70. 



