THE KEFLEX ACTIONS OF THE SPINAL CORD. 575 



We have already seen that the action of such nervous centres as the respi- 

 ratory and vasomotor centres, which frequently, at all events, is of a reflex 

 nature, may be either inhibited or augmented by afferent impulses. The 

 micturition* centre in the mammal, which is also largely a reflex centre, may 

 be easily inhibited by impulses passing downward to the lumbar cord from 

 the brain, or upward along the sciatic nerves. In the case of dogs, whose 

 spinal cord has been divided in the thoracic region, micturition set up as a 

 reflex act by simple pressure on the abdomen or by sponging the anus is at 

 once stopped by sharply pinching the skin of the leg. And it is a matter 

 of common experience that in man micturition maybe suddenly checked by 

 an emotion or other cerebral event. The erection centre in the lumbar 

 cord, also in large measure a reflex centre, is similarly susceptible of being 

 inhibited by impulses reaching it from various sources. And, indeed, 

 many similar instances of the inhibition of reflex movements might readily 

 be quoted. 



Several apparent instances of the inhibition of reflex acts are not really 

 such ; in these cases all the nervous processes of the act may take place in 

 their entirety and yet fail to produce their effect on account of a failure in 

 the muscular part of the act. Thus, when we ourselves by an effort of the 

 will stop the reflex movements which otherwise would be produced by 

 tickling the soles of the feet, we achieve this to a large extent by throwing 

 voluntarily into action certain muscles, the contractions of which antagonize 

 the action of the muscles engaged in carrying out the reflex movements. 

 But it may be doubted, even in these cases, whether inhibition is always or 

 wholly to be explained in this way ; and certainly in very many instances 

 of reflex inhibition no such muscular antagonism is present, and the reflex 

 act is checked at its nervous centre. 



When the brain of a frog is removed, and the effects of shock have 

 passed away, reflex actions are developed much more readily and to a much 

 greater degree than in the entire animal, and in mammals also reflex excita- 

 bility has been observed to be increased by removal of the cerebral hemi- 

 spheres. This suggests the idea that in the intact nervous system the brain 

 is habitually exerting some influence on the spinal cord, tending to prevent 

 the normal development of the spinal reflex actions. And we learn by ex- 

 periment that stimulation of certain parts of the brain has a remarkable 

 effect on reflex action. If a frog, from which the cerebral hemispheres have 

 been removed (the optic lobes, bulb, and spinal cord being left intact), be 

 suspended by the jaw, and the toes of the pendent leg be from time to time 

 dipped into very dilute sulphuric acid, a certain average time will be found 

 to elapse between the dipping of the toe and the resulting withdrawal of the 

 foot. If, however, the optic lobes or optic thalami be stimulated, as by 

 putting a crystal of sodium chloride on them, it will be found on repeating 

 the experiment, while these structures are still under the influence of the 

 stimulation, that the time intervening between the action of the acid on the 

 toe and the withdrawal of the foot is very much prolonged. That is to say, 

 the stimulation of the optic lobes has caused impulses to descend to the cord, 

 which have there so interfered with the nervous processes engaged in carry- 

 ing. out reflex actions as greatly to retard the generation of efferent impulses, 

 or, in other words, has inhibited the reflex action of the cord. And similar 

 results may be obtained in mammals by stimulating certain parts of the 

 corpora quadrigemina, which bodies are homologous to the optic lobes of 

 frogs. From this it has been inferred that there is present in this part of 

 the brain a special mechanism for inhibiting the reflex actions of the spinal 

 cord, the impulses descending from this mechanism to the various centres of 

 reflex action being of a specific inhibitory nature. But, as we have already 



