SECTION V 

 FUNCTIONS OF THE NERVE-CELL 



WHEN a unicellular organism, containing a single nucleus, is cut into two 

 parts, both continue to live for some time, each performing active move- 

 ments and evincing all the phenomena which we associate with activity 

 and therefore with destructive katabolism. For the continued existence 

 of a cell the processes of constructive metabolism, or anabolism, must take 

 place pari passu with those of disintegration, and for this the presence of 

 the nucleus is necessary. Hence, in a few days, the half cell with the nucleus 

 has repaired its loss and become once again a normal individual, whereas 

 the half without a nucleus undergoes degeneration and death. The axon 

 of a nerve-cell can be regarded as analogous to a long pseudopodium of an 

 amoeba. Like this, if cut away from that part of the cell containing the 

 nucleus, though capable for a time of discharging its active function of 

 propagation of excitatory impulses, yet it finally dies, death of the nerve 

 fibre occurring in the mammal within three to five days after separation 

 of the axon from the cell. Every nerve- cell therefore may be looked upon 

 as a trophic centre of the nerve fibre proceeding from it as well as of the 

 medullary sheath, which is practically a product or secretion of the axis 

 cylinder. But has the nerve-cell any more important functions to dis- 

 charge ? It has long been customary to endow the nerve- cell with all 

 the properties which are distinctive of a nervous system, and to ascribe 

 to it the active part in the origination of automatic actions, in the reflection 

 of afferent impulses, and in the supply of energy to all nervous processes. 

 That the passage of impulses through the nerve-centres requires the ex- 

 penditure of energy by these centres can be proved in various ways. In 

 the first place, we have the fact that in all nervous systems, at any rate of 

 the higher animals, arrangements are made for their free supply with oxygen 

 Very short deprivation of oxygen causes a complete block throughout the 

 system, in many cases preceded by a short period of increased excitability 

 or ease of transmission. If, in the rabbit, the thoracic aorta be clamped 

 for a few minutes, the hind limbs become paralysed, and if the obstruction 

 be continued for half an hour, there is widespread degeneration and death 

 of the cells with their fibres in the grey matter of the lumbar and sacral 

 cord. In the second place, the ready production of fatigue of the nervous 

 system points to a considerable using up of material as a condition of the 

 passage of nerve impulses. In many instances, moreover, an infinitesimal 

 stimulus travelling up a few nerve fibres may excite widespread activity 

 of the whole central nervous system with^the discharge of impulses along 



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