OF NERVE-CELLS. 



267 



all those experiments which have been brought to prove the dissimilari- 

 ty of electricity and the nervous agent, by intervening a piece of metal 

 between a section of a nervous trunk, it having been found that under 

 such circumstances the nervous influence does not pass. 



The physical condition upon which the activity of the nervous mech- 

 anism depends is the supply of arterial blood ; for, although Nervous activ 

 the nerve fibres never receive or are penetrated by blood cap- ity depends on 

 illaries-, these latter run in company with them in the nerv- artenal b 

 ous fasciculi. It would appear, however, that the supply of arterial blood 

 is of far less moment in the function of the nerve fibres than it is in that 

 of the nerve centres. This is shown by the limited supply given to the 

 former, and the abundant one to the latter ; by the comparative effect of 

 a stoppage of the blood circulation, in which case the action of the nerve 

 centres is instantly arrested, whereas that of the fibres may continue for 

 a long time. On the whole, there are strong reasons for believing that 

 the conductibility of the nerve fibres is as purely physical as is that of a 

 metal wire, and that the supply of blood that they receive is only for the 

 purpose of maintaining their construction in a perfect state. 



We have stated that there are nerves the functions of which are essen- 

 tially different, such as the centripetal .or sensory, and the identification 

 centrifugal or motor. The identification of the class to of the class of 

 which a nerve under examination belongs, may sometimes 

 be made by examining its manner of distribution, or its ganglionic con- 

 nection ; sometimes by experiment, by making a section and irritating 

 the cut extremities. In these cases, however, caution has to be exer- 

 cised in coming to a conclusion. 



FUNCTIONS OF NERVE-CELLS. 



.The neTvous fibres having for their duty the conduction of external 

 impressions and the transmission of nervous influences, the Function of 



Fig. 121. nerve-cells or nerve vesicles - 



^ ' ' vesicles are for the re- 



ception of those impres- 

 sions and the origination 

 of those influences. The 

 nerve centres or ganglia 

 are made up of vesicles, 

 granules, and nerve-tubes 

 ft conjointly. 



Fig. 121, dorsal gan- 

 glion of the sympathetic 

 nerve of a mouse. $, b, 



Dorsal ganglion of mouse. COl'ds of Connection With 



