710 FUNCTIONS OF THE SPINAL CORD [CH. XL1X. 



foot is pinched it will draw the foot away; if left perfectly quiet it 

 remains motionless. 



The muscular response that follows an excitation of the surface 

 is purposive and constant, the path along which the impulse is pro- 

 pagated being definite. 



Under certain abnormal conditions, however, the propagation of 

 the impulse in the cord is widespread, the normal paths being, as it 

 were, broken down. This is seen in the convulsions that occur on 

 slight excitation in animals or men who have suffered from profuse 

 haemorrhage, or in the disease called lockjaw or tetanus. Such a 

 condition is easily demonstrable in a brainless frog under the influence 

 of strychnine: after the injection of a few drops of a 1 per cent, 

 solution under the skin, cutaneous excitation no longer produces co- 

 ordinated responses, but paroxysms of convulsions, in which the frog 

 assumes a characteristic attitude, with arms flexed and legs extended. 



Spreading of reflexes. If one lower limb is excited, it is that limb 

 which responds : if the excitation is a strong one it will spread to the 

 limb of the opposite side, and if stronger still, to the upper limbs also. 



Pfliiger taught that the direction of irradiation within the spinal 

 cord was always upwards. Sherrington has shown that this is not 

 so, and has discovered many descending paths (see p. 659). 



Cumulation of reflexes. This is well illustrated by Turck's method. 

 If a number of beakers of water are prepared, acidulated with 1, 2, 

 4, etc., parts of sulphuric acid per 1000, and the tips of the frog's 

 toes are immersed in the weakest, the frog at first takes no notice of 

 the fact, but in time the cumulation or summation of the sensory 

 impulses causes the animal to withdraw its feet. If this is repeated 

 with the stronger liquids in succession, the time that intervenes before 

 the muscles respond becomes less and less. This method also serves 

 to test reflex irritability when the frog is under the influence of 

 various drugs. 



Inhibition of reflexes. If, instead of the whole brain, the cerebrum 

 only is destroyed, and the optic lobes are left intact, response to 

 excitation is much slower, the influence of the remaining part of the 

 brain inhibiting the reflex action of the cord. Or if in doing the 

 experiment with acid just described the toes of the other foot are 

 being simultaneously pinched, the response to the acid is delayed. 



This influence of the brain on- the cord is also illustrated in man, 

 by the fact that a strong effort of the will can control many reflex 

 actions. It is, for instance, possible to subdue the tendency to 

 sneeze ; if one accidentally puts one's hand in a flame, the natural 

 reflex is to withdraw it : yet it is well known that Cranmer, when being 

 burnt at the stake, held his hand in the flames till it was consumed. 



After the spinal cord has been divided by injury or disease in the 

 thoracic region, the brain can no longer exert this controlling action ; 



