1/2 PHYSIOLOGY OF NERVE. [CH. xv. 



from the study of embryology, that the nerve-fibres of the 

 anterior root are connected to the nerve-cells within the spinal 

 cord, while the posterior root-fibres are connected to the cells 

 of the spinal ganglia; or, to put it another way, the trophic 

 centres which control the nutrition of the nerve-fibres are situated 

 within the cord for the anterior roots, and within the spinal 

 ganglia for the posterior roots. 



Changes in a Nerve during Activity. 



When a nerve is stimulated, the change produced in it 

 is called a nervous impulse ; this change travels along the 

 nerve, and the propagation of some change is evident from the 

 effects which follow : sensation, movement, secretion, &c. ; but in 

 the nerve itself very little change can be detected. There is no 

 change in form; the most delicate thermo-piles have failed to detect 

 any production of heat, and we are also ignorant of any chemical 

 changes. The only alteration which can be detected as evidence 

 of this molecular change in a nerve is the electrical one. Healthy 

 nerve is iso-electric, but during the passage of a nervous impulse 

 along it there is a very rapid diphasic variation, which travels 

 at the same rate as the nervous impulse. This is similar to the 

 diphasic change in muscle, which we have already studied, and 

 can be detected in the same way (see further, p. 188). 



Velocity of a Nerve Impulse. 



A nervous impulse is not electricity ; compared to that of electri- 

 city its rate of propagation is extremely slow. It has been measured 

 in motor-nerves as follows : a muscle-nerve preparation is made 

 with as long a nerve as possible ; the nerve is stimulated first as 

 near to the muscle, and then as far from the muscle, as possible. 

 The moment of stimulation and the moment of commencing con- 

 traction is measured by taking muscle tracings on a rapidly moving 

 surface in the usual way, with a time-tracing beneath. The con- 

 traction ensues later, when the nerve is stimulated at a distance 

 from the muscle, than in the other case, and the difference in the 

 two cases gives the time occupied in the passage of the impulse 

 along the piece of nerve, the length of which can be easily measured. 



A similar experiment can be performed on man by means of the 

 transmission myograph (see p. 129). If a tracing of the contrac- 

 tion of the thumb muscles is taken, the two stimuli may be 

 successively applied through the moistened skin, first at the 

 brachial plexus below the clavicle, and secondly, at the median 

 nerve at the bend of the elbow. 



