CHAP, i.] THE SPINAL COED. 699 



to each other, being so " coordinated " that the movement, the 

 result of the contractions, secures some end, either apparent or 

 real. Thus when the afferent fibres distributed to a small, it 

 may be a very small, area of the skin of the flank are stimulated 

 by placing on the area a small piece of paper soaked in dilute 

 acid, the hind leg of the same side, by a series of flexions and 

 extensions, repeatedly sweeps over the spot with the foot, the 

 purpose of the movement, namely, to brush off the cause of 

 irritation, being obvious. For the carrying out of any reflex 

 movement, simple or complex, some portion of the grey matter 

 of the cord must intervene between the afferent and efferent 

 fibres. In the simpler movement, spoken of above, as carried 

 out by a single segment of the cord, we may perhaps suppose 

 that the arborescent terminations of the afferent fibres impinging 

 on the nerve-cells giving off the efferent fibres, supply all the 

 nervous mechanism which is needed. In the more complex 

 and more ordinary movements, the mechanism must be propor- 

 tionately more complex ; and we may perhaps conclude that in 

 these other cells of the grey matter intervene between the ter- 

 mination of the afferent fibre, and the cell whose axis-cylinder 

 process is the efferent fibre. In any case we may venture to 

 speak of a nervous mechanism within the cord as the means by 

 which a reflex movement is carried out. 



The exact working of such a mechanism in each particular 

 case, and so the character of the movement carried out, is deter- 

 mined by various causes. It is in part determined by the charac- 

 ters of the afferent impulses. Simple nervous impulses generated 

 by the direct stimulation of afferent nerve fibres generally evoke 

 as reflex movements merely irregular spasms in a few muscles ; 

 whereas the more complicated differentiated sensory impulses 

 generated by the application of the stimulus to the skin, readily 

 give rise to large and purposeful movements. It is easier to 

 produce a complex reflex action by a slight pressure on or other 

 stimulation of the skin than by even strong induction-shocks 

 applied directly to a nerve trunk. If, in a brainless frog, the 

 area of skin supplied by one of the dorsal cutaneous nerves be 

 separated by section from the rest of the skin of the back, the 

 nerve being left attached to the piece of skin and carefully 

 protected from injury, it will be found that slight stimuli applied 

 to the surface of the piece of skin easily evoke reflex actions, 

 whereas the trunk of the nerve may be stimulated with even 

 strong currents without producing anything more than irregular 

 movements. 



459. The character of the movement forming part of a 

 reflex action is also influenced by the intensity of the stimulus. 

 A slight stimulus, such as gentle contact of the skin with some 

 body, will produce one kind of movement ; and a strong stim- 

 ulus, such as a sharp prick applied to the same spot of skin, will 



