5i8 SCIENCE PROGRESS. 



becomes the skein of long cell processes (nerve-fibres), and 

 of short cell processes ; but, just as in the complicated 

 woof of a great loom, so here, the nerve-fibres and short 

 processes, constituting the living threads, remain distinct 

 from each other though woven into close proximity. 



Each up nerve-fibre entering the spinal cord divides 

 into two, one branch descending ends near its entry, the 

 other proceeds further to end in various regions nearer the 

 cephalic portions of the system ; some of these approach 

 the start of one down line, some that of another, but the 

 gaps always exist between the ending of the one and the 

 commencement of any new line. 



It is more difticult to describe, in general language 

 which shall not be too technical, the essential features, not 

 of nerve-structure but of nerve-activity. 



The experimental study of the activity of isolated nervous 

 structures has revealed three aspects of their vital pheno- 

 mena, ( I ) Living nerves are in a molecular condition which 

 is readily upset by a slight change in their surroundings ; 



(2) they are so constituted that after such an upset they 

 rapidly recover the old position of molecular equilibrium : 



(3) finally, the altered state due to the upset, although thus 

 rapidly reconstituted, is passed on from the seats of dis- 

 turbance to such neighbouring portions as lie in immediate 

 structural and physiological continuity with the part that was 

 originally upset. The first aspect, i.e., the capacity to be 

 thus upset is termed nerve-excitability; the second, the upset 

 itself, the excitatory state ; the third, the propagation of the 

 upset, is termed nerve-transmission and such transmission 

 constitutes what may, for brevity, be called the passage of 

 a nervous impulse. This nervous impulse is thus compelled 

 to travel along definite routes just as a flame is compelled to 

 travel along a fuse, or a train along a railway line, and this 

 is due to the fact that such travelling is the successive 

 awakening of the line of communication, i.e\ of portions of 

 definite nerve-fibres. If now the long fuse or nerve-fibre 

 is fired by a local change, in the latter case a change of a 

 sensory end organ, and the nervous impulse thus started 

 travels up the fibre, what may happen when it reaches such 



