NERVOUS SYSTEM. 329 



or tracts of nervous matter (as we learn from the celebrated labours 

 of Sir Charles Bell) " has its peculiar endowment, inde- 

 pendently of the others which are bound up along with it; and 

 this same endowment it continues to have throughout its whole 

 length." 



Elasticity. — It has been generally supposed, that nerves are 

 inelastic in themselves, and that any extension or contraction 

 they admitted of, arose from the elasticity of their component 

 cellular membrane. Sir E. Home, however, has proved, by some 

 ingenious experiments, that they possess a power of retraction 

 when divided in the living body : a circumstance which, of late, 

 cannot altogether have escaped the observations of those who 

 have performed the operation of neurotumj/. This retraction 

 does not seem entirely to depend on any inherent contractility of 

 tissue, otherwise extension of the nerve would be a necessary 

 preparative ; on the contrary, it happens under the most com- 

 plete state of relaxation : an effect that will not place in the 

 dead subject under similar circumstances. 



Mode of Origin. — A nerve is said to have two extremities — 

 a cerebral, and a sentient : the former is that part by which it is 

 connected with the brain, or spinal marrow ; the latter, that by 

 which it terminates in the vifleous structures of the body. It 

 has been usual to say, that the* nerves arise, or have their be- 

 ginning, from the brain, though it would appear, from some 

 recent investigations into their composition and functions, that 

 we might, with equal propriety, regard them as deriving their 

 origin from the organs to which they are said to be distributed, 

 and as ending in the sensorium. Supposing, however, that they 

 do issue from the brain, there still remains unsolvable mystery 

 respecting their beginning or roots. Some nerves may be fol- 

 lowed through a tract or streak of pulpy matter, distinguishable 

 from the surrounding medullary substance, until at length we 

 lose the tract, from its vanishing in the cineritious substance. So 

 that, although we continue, for the sake of anatomical definition, 

 to assign certain parts of the brain as the beds or origins of 

 certain nerves, we are still, in truth, ignorant of the veritable 

 sources of their original or radical fibres. It would appear, 

 from many familiar facts (the result of experiments and patholo- 

 gical observations) that the nerves distributed to one side of the 

 body arise from or are connected with the opposite side of the 

 brain ; and if this be true, there m.ust be somewhere a decussa- 

 tion of them : if an injury be received on the left side of the skull, 

 the right side of the body will become paralytic ; an effect that 

 could not happen unless there was a ready communication be- 

 tween one side of the brain and the nerves of the opposite side of 

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