GENERAL FEATURES OF NERVOUS TISSUES. 131 



In a reflex action, on the other hand, the movements called forth by the 

 same stimulus may be in one case insignificant and in another violent and 

 excessive, the result depending on the arrangements and condition of the 

 reflex mechanism. Thus the mere contact of a hair with the mucous mem- 

 brane lining the larynx, a contact which can originate only the very slightest 

 afferent impulses, may call forth a convulsive fit of coughing, in which a very 

 large number of muscles are thrown into violent contractions; whereas the 

 .same contact of the hair with other surfaces of the body may produce no 

 obvious effect at all. Similarly, while in the brainless but otherwise normal 

 frocr, a slight touch on the skin of the flank will produce nothing but a faint 

 flicker of the underlying muscles, the same touch on the same part of a frog 

 poisoned with strychnine will produce violent lasting tetanic contractions of 

 nearly all the muscles of the body. Motor impulses, as we have seen, travel 

 jilong motor nerves without any great expenditure of energy, and probably 

 without increasing that expenditure as they proceed ; and the same is appa- 

 rently the case with afferent impulses passing along afferent nerves. When, 

 however, in a reflex action afferent impulses reach the nerve centre, a change 

 in the nature and magnitude of the impulses takes place. It is not that in 

 the nerve centre the afferent impulses are simply turned aside or reflected 

 into efferent impulses ; and hence the name "reflex" action is a bad one. 

 It is rather that the afferent impulses act afresh, as it were, as a stimulus to 

 the nerve centre, producing according to circumstances and conditions either 

 a few weak efferent impulses or a multitude of strong ones. The nerve centre 

 may be regarded as a collection of explosive charges ready to be discharged 

 i\nd so to start efferent impulses along certain efferent nerves, and these 

 charges are so arranged and so related to certain afferent nerves, that afferent 

 impulses reaching the centre along those nerves may in one case discharge a 

 few only of the charges, and so give rise to feeble movements, and in another 

 <^ase discharge a very large number, and so give rise to large and violent 

 movements. In a reflex action then the number, intensity, character, and 

 distribution of the efferent impulses, and so the kind and amount of move- 

 ment, will depend chiefly on what takes place in the centre, and this will, in 

 turn, depend, on the one hand, on the condition of the centre, and, on the 

 other, on the special relations of the centre of the afferent impulses. At 

 the same time we are able to recognize in most reflex actions a certain rela- 

 tion between the strength of the stimulus, or the magnitude of the afferent 

 impulses and the extent of the movement or the magnitude of the efferent 

 impulses. 



We may add, without going more fully into the subject here, that in 

 most reflex actions a special relation may be observed between the part 

 stimulated and the resulting movement. In the simplest cases of reflex 

 action this relation is merely of such a kind that the muscles thrown into 

 action are those governed by a motor nerve which is the fellow of the sen- 

 sory nerve, the stimulation of which calls forth the movement. In the more 

 complex reflex action of the brainless frog, and in other cases, the relation 

 is of such a kind that the resulting movement bears an adaptation to the 

 stimulus; the foot is withdrawn from the stimulus, or the movement is cal- 

 culated to push or wipe away the stimulus. In other words, a certain 

 purpose is evident in the reflex action. 



Thus in all cases, except perhaps the very simplest, the movements called 

 forth by a reflex action are exceedingly complex compared with those which 

 result from the direct stimulation of a motor trunk. 



98. Automatic actions. Efferent impulses frequently issue from the 

 brain and spinal cord and so give rise to movements without being obviously 

 preceded by any stimulation. Such movements are spoken of as automatic 



