302 THE BIOLOGY OF THE FROG chap 



regions in front of and behind the point stimulated. The 

 stronger the stimulus, the greater is the part of the body 

 involved in the response. A weak stimulus applied to the 

 foot produces movement only in the member stimulated, 

 while a much stronger stimulus may cause a movement of the 

 opposite leg as well as other parts of the body. If acid be 

 placed on one side of the body and the leg of that side held, 

 the leg on the opposite side is sometimes brought around 

 to wipe the acid away. This apparently intelligent act is 

 obviously dependent upon the passage of impulses from one 

 side of the cord to the other. And it requires a stronger 

 stimulus to bring it about than is necessary to produce 

 a simple unilateral reflex. 



These reflex actions of the spinal cord are of a purposive 



. nature ; they bring the limbs away from injurious stimuli 

 and remove irritating substances from the body. They are 



, dependent upon the organization of the animal, the neuro- 

 muscular mechanism being such that a stimulus to any part 

 of the body brings about the appropriate actions for remov- 

 ing or escaping from the source of injury. 



The reflex actions of a frog may be checked or modified 

 by impulses from the brain. A frog with its brain in con- 

 nection with the cord will not respond to stimulation in the 

 same regular and unvaried manner as a brainless individual. 

 Its actions are much less mechanical and more spontaneous 

 and uncertain of prediction. The influence of the brain is 

 rendered possible by means of nerve trunks which pass from 

 the brain down the spinal cord and form connections with 

 the neurones of the spinal nerves. If the anterior end 

 of the spinal cord is strongly stimulated at the same time a 

 stimulus is applied to the foot, the withdrawal of the latter 

 may be entirely prevented. In this way the ordinary reflex 

 actions of the cord are continually checked and modified by 

 impulses from the higher nervous centers. 



