96 INFLUENCE OF NERVES 



variety of nerves than others double their size, but 

 with more simple functions. 



Muscular power is (other circumstances being 

 equal) proportioned ta the size of the muscle ; but 

 it often happens that great power is required where 

 bulk of muscle would be inconvenient or cumber- 

 some. In such cases, it is supplied with an in- 

 creased endowment of nervous filaments, which 

 make up by the strength of stimulus what the mus- 

 cle wants in bulk of fibre. Many birds, for example, 

 require great muscular power to sustain them in 

 their long and rapid flights through the air, and owe 

 its possession chiefly to the strong stimulus im- 

 parted to moderate-sized muscles by large nerves, 

 which add nothing, or next to nothing, to their 

 weight ; whereas, had the greater power been ob- 

 tainable only from an augmentation of fleshy fibres, 

 the consequent addition of weight would, from the 

 greatly increased difficulty the animal must have 

 felt in raising and sustaining itself in the air, have 

 gone far to counterbalance any advantage gained on 

 the side of power. But in fishes, which float with- 

 out effort in their own element, size produces no 

 such inconvenience, and their strength, accordingly, 

 is made to depend more on the volume of the mus- 

 cle than on its nervous endowment, showing a 

 beautiful adaptation to the mode of life and wants 

 of the animal. 



As voluntary motion depends as much on nervous 

 stimulus as on muscular agency, it happens that 

 whatever interrupts the action of the nerves puts a 

 stop to motion as effectually as if the muscular fibre 

 itself were divided. Injuries and diseases of the 

 brain, whence the will emanates, are well known to 

 be accompanied with palsy, or want of power in the 

 muscles, although in their own structure the latter 

 remain sound. Sleep and narcotics, too, suspend 

 voluntary motion, solely in consequence of their 

 action on the nervous system. Ardent spirits, in 



