CH. XXIX.] THE PHYSIOLOGY OF MIJSTD. 19 



that usuallv contract under a stimulus transmitted through 

 the nerves, is not, however, a fvindamental diflerence. The 

 nature of the stimulus is the same in both cases ; it 

 originates in the same way, and affects the muscles in 

 the same way. Between the case of a muscle that con- 

 tracts from the stimulus of a foreign body coming into 

 contact with the muscle itself, and one that contracts 

 from the stimulus which the nerves transmit to it when 

 a similar body comes into contact with the extremity of a 

 nerve in some other part of the organism, there is probably 

 not much more difference than there is between the act of 

 ringing a bell in the hand and the act of ringing it at 

 the end of a bell-wire ; or, what is a better comparison, 

 between decomposing water by the action of sulphuric 

 acid on zinc while the acid and zinc are in the same 

 vessel with the water, and effecting the same decomposi- 

 *tion by arranging the acid and zinc in a voltaic battery, 

 and sending the liberated energy, in the form of a voltaic 

 current, along a wire to a separated vessel where the water 

 is to be decomposed. 



The nervous system is developed by differentiation out Nervous 

 of the muscular system, and in its first and lowest deve- system de- 



, . . veloped 



lopment appears to have no vestige of sensation and no out of 

 other function than that of transmitting stimuli to the 'j'^^'^^'^^'"- 

 muscles. The muscular tissue of the Hydrozoa, as stated pi-imaiy _ 

 above, transmits a stimulus but slowly. ISTervous tissue, on to'traus'^ ^^ 

 the contrary, transmits it with a rapidity which is practi- ^}^ 

 cally instantaneous ; and this, of course, makes the motions to the 

 more rapid, and the whole life more energetic, in those °^^^''°^®'*" 

 animals which have a nervous system than in those which 

 are without one. 



1 It is, however, a very remarkable, and at present an 

 inexplicable fact, that the nervous system is not, in any Nervous 

 animal whatever, so simple as might be thought from the system 

 toregomg account of its functions. The nerve-fibres are simple, 

 in no case directly laid on, as it were, like telegraph wires, 



1 All the facts and opinions respecting the anatomy and physiology of 

 the nervous system stated in this chapter are taken from Carpenter's 

 Human Physiology, except where I advance any opinion as my own. 



C 2 



