THE NERVOUS SYSTEM 457 



3. Reflex inhibition exhibits itself in respiration, stepping, eyeball 

 movements, and the reciprocal innervation of antagonistic muscles, 

 the underlying principle being that by means of reflex inhibition 

 the contraction of a muscle causes relaxation of its antagonist. 



4. Inhibition is the main means by which rhythmic reflexes are 

 produced. For example, the rhythmic reflex in stepping being excited, 

 its execution produces a stimulus, which brings into activity a reflex 

 inhibition ; this cuts the rhythmic reflex short. The muscle having 

 ceased to contract, the inhibition is removed, and the original stimulus 

 is once more in undisputed possession of the field, and so rhythmicity 

 is maintained. 



5. On the withdrawal of inhibition, there is a great augmentation 

 of activity in the inhibited centre. This Sherrington describes as 

 post-inhibitory rebound ; this rebound favours the change from flexion 

 to extension and vice versa. He regards it as responsible for one of 

 the two phases of such diphasic muscular reflexes as stepping, mastica- 

 tion, and respiration. It is evident that when one or more pairs of 

 legs are concerned in locomotion, some of the limbs are in a condition 

 of flexion, the others are in extension ; and the change from one phase 

 to the other is brought about by post-inhibitory rebound. 



6. Strychnine converts reflex inhibition into reflex excitation, and 

 the toxins of tetanus and rabies have the same effect. These agents, 

 as pointed out by Sherrington, ' work havoc ' with the elemental 

 co-ordination of the skeletal muscles, changing reciprocal innervation 

 into identical innervation. Hence, in tetanus patients, the greater 

 the effort to open the jaw, the more tightly is it closed ; and in the 

 hydrophobia patient the greater the attempt made by the sufferer 

 to inhibit his inspiration in order to swallow, the deeper and more 

 prolonged the inspiration which ensues, inhibition ■ being converted 

 by the virus into excitation.' 



7. Chloroform and ' fatigue' strengthen inhibition. An inhibitory 

 reflex stimulus which is ineffective on a normal centre, when that 

 centre is fresh, becomes effective when the centre is under chloroform, 

 or when the active driving of the centre has been pushed for a 

 little time. 



In complex reflexes many stimuli are at work together, and 

 co-operate harmoniously for a co-ordinate result. In standing, 

 walking, running, etc., very important sources of the reflex lie 

 in the muscles and joints of proximal parts of the limb — namely, 

 in the joints of the hip and stifle and the great muscles acting 

 on them. These joints and muscles are liberally supplied with 

 afferent nerves. The importance of these as sources of the 

 reflex of stepping is indicated by several facts. In the first 

 place, a dog or cat is found still to walk well when the foot 

 reflex is cut off in all four feet by division of the nerves, both 

 superficial and deep. In the second place, when the spinal dog 

 is lifted so that its limbs do not touch any solid support what- 

 ever, reflex walking and galloping are performed, although the 

 limbs are stepping wholly in the air. But to excite this reflex of 

 walking in the air, it is necessary that the limbs hang down. 

 The reflex ceases if the dog be inverted, when gravity no longei 

 is acting on the joints and muscles as it does in the position 



