882 THE CENTRAL NERVOUS SYSTEM 



reflexes is afforded when either the scratch reflex or the flexion 

 reflex is induced and caused to interrupt the other while it is pro- 

 ceeding. The transition e.g., from flexion to scratch reflex is 

 made without any period of confusion. Thus, if the scratch reflex 

 has been induced and is being executed, and the foot is then pain- 

 fully stimulated, the scratch reflex immediately ceases, and the 

 flexor reflex takes its place. When the flexor reflex has termin- 

 ated, the scratch reflex may be resumed. The same holds good for 

 other antagonistic reflexes. In many cases the avoidance of con- 

 fusion is due to the inhibition of the first reflex, or often to inhibition 

 of the set of muscles which were active in the first reflex combined 

 with excitation of their antagonists (so-called interference). It is 

 obvious that this is an adaptation of great importance. 



Influence of the Brain on the Spinal Reflexes. The spinal reflexes 

 can be influenced by impulses descending from the higher centres. 

 For (a) it is a matter of common experience that a reflex movement 

 may be to a certain extent controlled, or prevented altogether by an 

 effort of the will, and it is worthy of remark that only movements 

 which can be voluntarily produced can be voluntarily inhibited. 

 (b) Long-continued muscular contractions may be caused in animals 

 after removal of the cerebral hemispheres by stimulation of afferent 

 nerves for example, by scratching the mucous membrane of the 

 mouth in a ' brainless ' frog or Necturus. (c) By stimulation of 

 certain of the higher centres reflex movements which would other- 

 wise be elicited may be suppressed or greatly delayed. If the 

 cerebral hemispheres are removed from a frog, and one leg of the 

 animal dipped into dilute acid, a certain interval, the (uncorrected) 

 reflex time, will elapse before the foot is drawn up (p. 959). If, 

 now, a crystal of common salt be applied to the optic lobes, or 

 corpora bigemina,or the upper part of the spinal cord, and the experi- 

 ment repeated, it will be found either that the interval is much 

 lengthened or that the reflex disappears altogether. Stimulation 

 of the optic lobes relaxes the reflex sexual embrace of the male frog 

 when it is present. From such experiments it has been concluded 

 that centres which can inhibit the spinal reflexes are situated in the 

 thalamus, the corpora bigemina and the medulla oblongata of the 

 frog. In mammals, also, there is evidence of the existence of 

 mechanisms in the brain, the excitation of which diminishes the 

 reflex excitability of the cord. For example, stimulation of the 

 frontal convolutions in the dog causes a diminution in the height 

 of reflex contractions of the limbs. Strong stimulation of an 

 afferent nerve may abolish or delay a reflex movement which is 

 being elicited through other receptors, (d) If such inhibitory 

 mechanisms exist, it is to be supposed that elimination of the brain 

 will render it easier to elicit reflexes from the cord. Experiment 

 shows that this is actually the case. An animal like a frog responds 



