NERVOUS SYSTEM IN GENERAL. 29 



some analogous organ (adjuncts of the peripheral surfaces) ; 

 it is transmitted by a centripetal fibre to a central globule, 

 which reflects this by a centrifugal fibre to another organ 

 more or less peripheral, as, for instance, a muscle whose con- 

 traction may be thus effected, or to a gland which then pours 

 out its secretion. 



Thus fibres perform their function of carrying the excita- 

 tion towards a globule, or of transmitting it from the globule 

 to the periphery; hence the names centripetal or sensitive 

 given to the former nerves, and centrifugal or motory to the 

 latter. This name should indicate merely that this is the 

 sense in which the function of the fibre is manifested to us, 

 but no essential difference between centripetal and centrif- 

 ugal filaments are intended, as we shall soon see that direct 

 experiments demonstrate the contrary. 



The office of the globule is to favor the transmission of 

 the excitation from one to another fibre ; oftentimes, indeed, 

 the first globule reflects its action, by commissural fibres, 

 upon one or several other globules which can turn the action 

 in a different direction again, either directly upon a centrif- 

 ugal fibre, properly so called, or upon some fresh nerve 

 globules : the globular elements can even absorb or enfeeble 

 the action, or even store it up, as it were, in a latent state, 

 and send it off only at another time, when influenced by 

 new excitations. Hence we see that reflex centres present 

 very complicated phenomena, becoming at one time centres 

 of diffusion, and again of co-ordination of movements, of 

 memory, etc. ; these centres can be also the seat of sensation 

 for the peripheral excitations. 



Leaving out of mind the central phenomena that are 

 difficult of analysis, we see that the office of the nerves is 

 essentially that of conduction. Now what constitutes con- 

 duction, and what is the peculiar phenomenon by which it is 

 characterized ? For a long time this was supposed to resem- 

 ble and partake of the nature of the electric current ; but at 

 the present time it is proved that the nerve influx has nothing 

 to do with electricity. In the first place its rapidity of 

 propagation has been calculated to be 28 to 30 metres to the 

 second, a very different rate from that of the electric cur- 

 rent, and even this varies with the temperature of the nerve ; 

 according to Helmholtz, in frog's nerve cooled to a tempera- 

 ture of the freezing point of water (O p c.), the rapidity of the 

 nerve agent is but one-tenth of what it is at 15 or 20 

 higher. Again, when the nerve performs its functions, in- 



