SPECIFIC ENERGIES OF NERVE-CELLS. 283 



9. From all these details it is very evident that 

 the nerve-fibres are homogeneous the one with the 

 other, and that the difference in their effects is to be 

 referred to their connection with nerve-cells of varied 

 form. This seems, however, to be opposed to the fact 

 that the different sense-nerves are irritable by quite 

 different influences, and each of them only by quite 

 definite influences the nerve of sight by light, the 

 nerve of hearing by sound, and so on. It would, how- 

 ever, be a mistake to infer from this that the nerve of 

 sight is really different from the nerve of hearing. If 

 the matter is examined more closely, it appears that 

 the nerve of sight cannot be excited by light. The 

 strongest sunlight may be allowed to fall on the nerve 

 of sight without producing excitement. It is not the 

 nerve, but a peculiar terminal apparatus in the retina 

 of the eye with which the nerve of sight is connected, 

 which is sensitive to light. The case of the other 

 sense-nerves is similar; each is provided at its peri- 

 pheric end with a peculiar receptive apparatus, which 

 can be excited by definite influences, and which then 

 transmits these influences to the nerves. On the 

 difference in the structure of these terminal apparatus 

 depend which influences have the power of exciting 

 them. When the excitement has once entered the 

 nerve it is always the same. That it afterward elicits 

 different sensations in us, depends again on the character 

 of the nerve-cells in which the nerve-fibres end. Sup- 

 posing that the nerves of hearing and of sight of a 

 man were cut, and the peripheric end of the former 

 were perfectly united with the central end of the 

 latter, and contrariwise that the peripheric end of the 

 nerve of sight were perfectly united with the centra] 



