276 PHYSIOLOGY OF MUSCLES AND NERVES. 



from nerve-cell to nerve-cell, from each of which it 

 can repass into a motor fibre. From the length of 

 the time occiipied by the reflex irritant, it is to be 

 inferred that the transmission of the excitement has 

 to meet considerable resistance in the nerve-cells. 

 This resistance naturally increases with the number of 

 nerve-cells to be traversed, so that the transference of 

 a reflex action from a definite sensory fibre to different 

 motor nerve-fibres is not always equally difficult, and 

 is the more difficult the greater is the number of 

 the cells which lie between the two. All this agrees 

 with the facts found by experiment. It also explains 

 why, by certain influences, not only is the reflex trans- 

 ference rendered easier, but the passage of the excite- 

 ment to the most remote motor fibres is also rendered 

 peculiarly possible. The best known case of this is 

 poisoning by strychnine. This so greatly facilitates the 

 reflex transference that the slightest touch on any point 

 of the skin, or even the disturbance caused by a breath, 

 is sufficient to throw all the muscles of the body into 

 violent reflex tetanus. 



As each excitement of a sensory fibre which reaches 

 the nerve-centre can give rise to a conscious sensation, 

 the spread of the excitement within the centre must 

 have the same efi'ect as would be the case if a larger 

 number of excitements of several sensory fibres reached 

 the centre simultaneously. This process, \vhich, how- 

 ever, only occurs in the case of strong excitements, 

 is called co-relative sensation. Sensation is caused 

 not only by the excitement of the nerve-cell directly 

 concerned, but also by the spread of the excitement 

 to the other nerve-cells. It may also be spoken of as 

 the radiation of the sensory irritant, because the excite- 



