INFLUENCE OF THE QUALITY OF THE STIMULUS. 825 



obvious response may be forthcoming. Yet, in many cases, that does not 

 make us for a moment disbelieve that there has been spread of nervous 

 reaction to the excitation. We say " it was felt, but caused no move- 

 ment." When movement occurs, it is interesting to note the diversity 

 of range it habitually exhibits, the slightest movement of one part being 

 associated with adjusted movements of others. The widest irradiation of 

 a spinal reflex is poverty-stricken compared with such habitual normal 

 reactions. Further, the incidence of the paralysis falls, as shown by the 

 course of the spinal irradiation, unequally upon different regions of the 

 musculature. The extensor muscles of the limbs, just as they suffer 

 most depression of tonus, seem also, at least in regard to the primary 

 phase of reaction, to be reflexly more paralytic than are the antagonistic 

 flexors. But observations on this point are still scanty under circum- 

 stances where the initial spinal shock has passed off. There is some 

 evidence that after a time the extensors of the hip recover much brisk- 

 ness of reaction even in the spinal mammal. 1 



In spinal reflexes, the course of irradiation, as compared with that 

 of normal reactions elicited by a similar external stimulus, can be 

 foretold with relative success. The limb on being touched will be 

 flexed at certain joints ; this can be said of the spinal animal with far 

 greater likelihood of its proving correct, than the normal animal; but even 

 under the most rigorous conditions the fatality of the reaction in 

 the spinal animal is far from absolute. The field of musculature pro- 

 voked to action can be predicted with greater certainty than can the 

 direction the movement will take. It cannot be definitely predicted that 

 the normal cat when touched on the planta will move the limb touched ; 

 a movement of the eye or head may follow, and none of the limb. In the 

 spinal cat, it can, however, be predicted with much certainty that such a 

 stimulus will cause a movement of flexion of that limb the skin of 

 which is stimulated. 



Influence of the quality of the stimulus The external stimuli 



which are excitant of naked nerves, namely, those known as " general " 

 nerve stimuli, e.g. mechanical, thermal, chemical, and electrical, are all 

 efficient to evoke spinal reflexes. The adequate stimuli for the cutaneous 

 end-organs are also efficient. Inasmuch as the end-organs have been 

 adapted and specialised for certain external stimuli, the threshold value 

 of the adequate stimuli is low as compared with that of the general 

 stimuli, and this is especially evidenced in their reflexigenous power. 

 This probably explains the fact first noted by Marshall Hall, 2 that the 

 stimulation of a nerve trunk is ceteris paribus less qualified to generate 

 a reflex action than is the stimulation of the skin, although the stimula- 

 tion of a nerve trunk represents the stimulation of a relatively large 

 sensifacient surface. 



At the same time it must be remembered that the " general " nerve 

 stimuli stand in peculiar relation to pain, at least to cutaneous pain ; 

 and further, that pathic stimuli seem peculiarly efficient for producing 

 spinal reflexes. The movement provoked may differ with the quality 

 of stimulus : thus, the tail of the eel is made to approach by a tactile 



1 The greater activity of flexors than extensors after spinal transection lias been noted 

 by Sanders-Ezn, Wundt. Lombard, and many others. But primary extension at hip is 

 not uncommon when time has been allowed for subsidence of shock. At the ankle-joints 

 (dog) the paraplegic posture often comes in the course of weeks to be rather over-extension 

 than over-flexion. 



5 " Memoirs on the Nervous System," London, 1837. 



