500 PRINCIPLES OF GENERAL PHYSIOLOGY 



the limbs is greatly influenced by stimuli from the receptors of the labyrinth 

 and the neck, and that this extensor tonus is associated with inhibition of 

 flexors. Now, in this particular case of reciprocal inhibition, it has been 

 shown by Magnus and Wolf (1913, p. 458), that the inhibitory component 

 cannot be reversed even by so large a dose of strychnine that convulsions 

 made further experiments impossible. These observations were made on triceps 

 brachii and on vasto-crureus isolated. Since it was shown by Sherrington that a 

 small dose of strychnine reverses the inhibitory component of reflexes from afferent 

 nerves of the limb observed, in which these same muscles are employed, it follows 

 that the same muscle in one reflex may respond with reversed inhibition (i.e., 

 excitation), and in another reflex with normal inhibition. Magnus and Wolf 

 rightly draw the conclusion that no " anatomical " scheme of connections can 

 explain this fact. It may be that there are two independent synapses with the 

 final common path, unequally sensitive to strychnine, or, in accordance with the 

 conclusion to which I was led by my observations on vasomotor reflexes, that 

 the drug acts on some intermediate synaptic membranes on the afferent side, 

 synapses which are not part of the path common to the two different reflexes. One 

 of these, that in the reflex arc of the afferent nerve from the limb itself, is more 

 sensitive to the drug than those of the posture reflexes from" labyrinth and neck. 

 But it seems that either hypothesis would suffice. 



It will be clear, in any case, what havoc strychnine and tetanus toxin must 

 play with reciprocal innervation in the organism. As Sherrington says (1905, 

 p. 296) : " The sufferer is subjected to a disorder of co-ordination which, though 

 not necessarily of itself accompanied by physical pain, must inflict on the mind, 

 which still remains clear, a torture inexpressibly distressing. Each attempt to 

 execute certain muscular acts of vital importance, such as the taking of food, is 

 defeated because from the attempt results an act exactly the opposite to that 

 intended. The endeavour to open the jaw to take food or drink induces closure of 

 the jaw, because the normal inhibition of the stronger set of muscles the closing 

 muscles is by the agent converted into excitation of them. Moreover, the 

 various reflex arcs that cause inhibition of these muscles not only cause excitation 

 of them instead, but are, periodically or more or less constantly, in a state of 

 hyper-excitement ; and yet attempt on the part of the sufferer to restrain, to 

 inhibit, their reflex reaction, instead of relaxing them, only heightens their 

 excitation further, and thus exacerbates a rigidity or a convulsion already in 

 progress." Sherrington thinks it probable that the action of the toxin of rabies 

 lies in a similar effect on the mechanisms regulating swallowing and respiration. 



INTERACTION OF REFLEXES 



Various reflexes use the same final common path for different purposes or for 

 similar purposes. Afferent arcs which use it for different purposes cannot have 

 possession of it simultaneously and they must take their turns, as it were. One 

 reflex may defer or cut short another. This takes place even if they are both 

 associated with excitation of the motor neurone, if they use the final common path 

 in a different way, as regards time relations, and so on. - Such cases are the flexion 

 reflex and the scratch reflex of the leg muscles. These are " antagonistic " 

 reflexes. " Allied " reflexes act together and frequently reinforce one another. 



The function of the receptors of the muscle itself, " proprio-ceptors " as we 

 shall learn to call them, is of importance in this process. According to Sherrington 

 (1906, p. 341, and 1909, 3, p. 155), their function is to cut short a reflex and 

 prepare the arc for another one. Thus, a normal muscle, excited to reflex 

 contraction, can be inhibited by stretching it, whereas, in a muscle deprived of 

 its proprio-ceptive afferent fibres by section of the dorsal roots, this cannot 

 be done. 



COMPOUND REFLEXES 



Certain reflexes may combine together to form a definite co-ordination, 

 which may be either simultaneous or successive. In the latter case, the result 



