486 ELEMENTARY PHYSIOLOGY less. 



pinched, the legs are suddenly drawn up and thrust out 

 again violently. 



If the flank be very gently stroked, there is simply a 

 twitching movement of the muscles underneath ; if it be 

 more roughly touched, or pinched, the-se twitching move- 

 ments become more general along the whole side of the 

 creature, and extend to the other side, to the hind legs, 

 and even to the front legs. 



If the digits of the front limbs be touched, these will 

 be drawn close under the body as in the act of clasping. 



If a drop of vinegar or any acid be placed on the top of 

 one thigh, rapid and active movements will take place in 

 the leg. The foot will be seen distinctly trying to rub off 

 the drop of acid from the thigh. And what is still more 

 striking, if the leg be held tight and so prevented from 

 moving, the other leg will begin to rub off the acid. 

 Sometimes if the drop be too large or too strong, both 

 legs begin at once, and then frequently the movements 

 spread from the legs all over the body, and tlie whole 

 animal is thrown into convulsions. 



Now all these various movements, even the feeblest 

 and simplest, require a certain conibination of muscles, 

 and some of them, such as the act of rubbing off" the acid, 

 are in the highest degree complex. In all of tliem, too, a 

 certain purpose or end is evident, which is generally 

 either to remove the body, or part of the body, from the 

 stimulus, from the cause of irritation, or to tln-ust away 

 the offending object from the body : in the more complex 

 movements such a purpose is strikingly apparent. 



It seems, in fact, that in the frog 's spinal coi'd there 

 are sets of nervous machinery destined to be used for a 

 variety of movements, and that a stimulus passing along 

 a sensory nerve to the cord sets one or the other of these 

 pieces of machinery at work. 



Thus one important function of the spinal cord is to 

 serve as an independent nervous centre, capable of origin- 

 ating combined movements upon the reception of the 



