508 PRINCIPLES OF GENERAL PHYSIOLOGY 



impulses, some inhibitory, and the discharge depends on the algebraic sum of l. 

 whole, as shown in double reciprocal innervation. 



When the relative intensities of the excitatory and inhibitory stimuli are very 

 nearly balanced, a rhythmic discharge results. In the case of the leg, this is 

 alternating on the two sides, so that stepping is produced. Explanations suggested 

 will be found in the text. 



The effects of strychnine and of chloroform in converting inhibition into excita- 

 tion, and vice versa, show themselves in reflexes dealt with by reciprocal innervation. 

 This action is not exercised on the final common path itself, but either at the 

 synapses of different neurones with it, or in some previous synapses, as in fact 

 follows from the seat of the inhibition itself, as stated above. The effect of the con- 

 version of inhibition into excitation by tetanus toxin in willed movements and 

 the suffering it causes is described in the text. 



When reflexes use the same final common path for different purposes, they 

 may either reinforce or inhibit each other. Even if they both excite, they may 

 not be able to use the motor neurones at the same time, if they use them in a 

 different way. The proprio-ceptors of a muscle are of use in cutting short one 

 reflex and preparing the arc for another. 



There are numerous compound reflexes, in which the result of one sets another 

 into play. 



The intermediate synapses of a reflex arc are comparatively easily fatigued, 

 whereas the motor neurones themselves are not so. This fact is of value in 

 preventing the occupation of a particular arc by one reflex for too long a time. 

 Recovery is fairly rapid. A reflex may cease, therefore, either from fatigue or 

 by inhibition, and there are differences between the two cases by which they can 

 be recognised. 



Reflexes from nocuous stimuli are prepotent. Their receptors are probably 

 free nerve endings. The stimulus required is comparatively large, as is appropriate 

 to the purpose of the reflex. 



The mechanism of autotomy in the crab is described in the text. 



The' difference between spinal reflexes and those in which the higher centres, 

 and especially the cerebral cortex, take part is the regularity of the former and 

 the ease with which the latter are modified or abolished by events in other parts of 

 the central nervous system. For this reason, Pavlov calls the former "uncon- 

 ditioned " and the latter " conditioned " reflexes. 



Notwithstanding this fact, Pavlov has devised methods by which conditioned 

 reflexes are amenable to experimental investigation and obtained many valuable 

 results. 



The two fundamental mechanisms involved are those of temporary association 

 and that of the " analysers." 



By the former, a stimulus presented several times in conjunction with an 

 unconditioned stimulus, such as food in the mouth, has ultimately the effect of 

 exciting salivary secretion when presented alone. It has become a conditioned 

 reflex and presents the opportunity for testing the effect of various other stimuli, 

 or events in the external world, on cerebral phenomena. 



The " Analysers " are the sense organs, together with their central connections. 



The production of a conditioned reflex may be expressed in one way by saying 

 that the reflex arc has taken on a new afferent neurone. But it must be 

 remembered that the connection with this neurone is very easily broken or 

 inhibited, and is modifiable in various ways. 



Inhibition is, in fact, the most common fact met with, as in the experiments 

 of Graham Brown and Sherrington on cortical stimulation, as described on page 

 480. It may even show itself as internal inhibition, when the proper consumma- 



