NERVOUS SYSTEM.— GENERAL CONSIDERATIONS. 209 



of labor, we might expect that there would be at least two cen- 

 tral cells— one to receive and the other to transmit impulses — 

 or at least that there should be some specialization among the 

 central cells ; and we shall have good reason later to believe 

 that this has reached a surprising degree in the highest ani- 

 mals. 



Moreover, it would be a great advantage if the termination 

 of the ingoing (afferent) nerve should not lie exposed on the 

 surface, but be protected by some form of cell that had also the 

 power to transmit to it the impressions received from without, 

 in a form suitable to the nature of the nerve and the needs of 

 the organism. 



So that a complete mechanism in its simplest form would 

 furnish: 1. A peripheral cell or nerve end-organ. 2. An affer- 

 ent or sensory nerve. 3. Two or more central cells. 4. An 

 efferent nerve, usually connected with — 5. A muscle or other 

 form of cell, the action of which may be modified by the out- 

 going nerve, or, as we should prefer to say, by the central 

 nervous cells through the efferent nerve. The advantages of 

 the principal cells being within and protected are obvious. 



When, then, an impression made on the peripheral cell is 

 carried inward, there modified, and results in an outgoing nerv- 

 ous impulse answering to the afferent one, giving rise to a mus- 

 cular contraction or other effect not confined to the recipient 

 cells, the process is termed reflex action. 



The great size, the multiplicity of forms, the distinct out- 

 line and lai'ge nuclei of nerve-cells, suggest the probability that 

 they play a very important part, and such is found to be the 

 case. Indeed, in some sense the rest of the nervous system may 

 be said to exist for them. 



Probably nerve-cells do sometimes act as mere conductors 

 of nervous impulses originating elsewhere, but such is their 

 lowest function. Accordingly, it is found that the nature of any 

 reflex action depends most of all on the behavior of the central 

 cells. 



It can not be too well borne in mind that nerves are con- 

 ductors and such only. They never originate impulses. 



The properties considered in the last chapter are common to 

 all kinds of nerves known ; and though we must conceive that 

 there are some differences in the form of impulses, these are to 

 be traced, not to the nerve primarily, but to the organ in which 

 it ends peripherally or to the central cells. 

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