182 REFLEX ACTIONS. [BOOK i. 



motor trunk. When the peripheral stump of a divided sciatic 

 nerve is stimulated with the interrupted current, the muscles of 

 the leg are at once thrown into tetanus, continue in the same rigid 

 condition during the passage of the current, and relax immediately 

 on the current being shut off. When the same current is applied 

 for a second only, to the skin of the flank of a brainless frog, the 

 leg is drawn up and the foot rapidly swept over the spot irritated, 

 as if to wipe away the irritation ; but this movement is a complex 

 one, requiring the contraction of particular muscles in a definite 

 sequence, with a carefully adjusted proportion between the amounts 

 of contraction of the individual muscles. And this complex move- 

 ment, this balanced and arranged series of contractions, may be 

 repeated more than once as the result of a single stimulation of the 

 skin. When a deep breath is caused by a dash of cold water, the 

 same co-ordinated and carefully arranged series of contractions is 

 also seen to result, as part of a reflex action, from a simple stimulus. 

 And many more examples might be given. 



In such cases as these, the complexity may be in part due to 

 the fact that the stimulus is applied to terminal sensory organs 

 and not directly to a nerve-trunk. As we shall see in speaking of 

 the senses, the impulses which are generated by the application of 

 a stimulus to a sensory organ are more complex than those which 

 result from the direct stimulation of a sensory nerve-trunk. Never- 

 theless, reflex actions of great if not of equal complexity may be 

 induced by stimuli applied directly to a nerve-trunk. We are 

 therefore obliged to conclude that in a reflex action, the processes 

 which are originated in the centre by the arrival of even simple 

 impulses along afferent nerves may be highly complex ; and that 

 it is the constitution and condition of the centre which determines 

 the complexity and character of the movements which are effected. 

 In other words, a centre concerned in a reflex action is to be 

 regarded as constituting a sort of molecular machinery, the 

 character of the resulting movements being determined by the 

 nature of the machinery set going and its condition at the time 

 being, the character and amount of the afferent impulses deter- 

 mining exactly what parts of and how far the central machinery is 

 thrown into action. 



Throughout the above we have purposely used the word 

 centre, avoiding the mention of nerve cells. But undoubtedly the 

 part of the spinal cord acting as centres of reflex action is situated 

 in the grey matter, which grey matter is characterised by the 

 presence of nerve cells; undoubtedly also the efferent fibres are 

 connected with the afferent fibres by means of cells, certainly by 

 the cells of the anterior cornu described in 99 and probably also 

 by other cells in the posterior cornu or elsewhere. So that a 

 reflex action is carried on undoubtedly through cells. But it does 

 not follow that a cellular mechanism is essential in the sense at all 

 events that the nuclei of the cells have anything to do with the 



