THE SPINAL CORD 233 



alcohol). In cold-blooded animals it increases with the 

 temperature. 



Inhibition of reflexes. 



1. Many reflexes can be inhibited by volition. But we 

 cannot voluntarily inhibit reflexes which are produced by 

 muscles which cannot be contracted voluntarily (e.g. the 

 contraction of the muscles of the uterus, contraction of the 

 pupil). 



2. There are special reflex inhibition mechanisms which 

 are not dependent upon volition. 



It is supposed that in man the centres of such mechanisms 

 lie in the ganglia of the brain, and that from it fibres pass 

 down the gray matter of the cord and act, in a manner still 

 unknown, upon the cells so that the reflex is inhibited. If 

 the function of these fibres is abolished by transverse section 

 of the spinal cord, the reflexes below the level of the section 

 are increased. 



In the frog reflex inhibition centres have been demon- 

 strated in the optic lobes, the stimulation of which prevents 

 the reflexes. 



3. A reflex brought about by the stimulation of a sensory 

 nerve can sometimes be inhibited by the simultaneous stimu- 

 lation of another sensory nerve. 



4. SPECIAL REFLEX CENTRES IN THE SPINAL CORD 



There are in the spinal cord centres for certain move- 

 ments which can be brought into activity reflexly. 



(1) In the cervical region are centres for the pupil reflex: 

 (a) A centre for the dilation of the pupil, in the upper 



part of the cervical cord ; 



() A centre for the constriction of the pupil, in the lower 

 part of the cervical cord. 



This pupil reflex serves to regulate the amount of light 

 entering the eye. For details see page 267. 



(2) In the lumbar region: 

 (a) Micturition centre. 



