CHAP, in.] GENERAL FEATURES OF NERVOUS TISSUES. 145 



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 arrange- 

 ments and condition of the central portion of the reflex mechanism. 

 Thus the mere contact of a hair with the mucous membrane 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 con- 

 tractions ; 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 frog, a slight touch on the skin 

 of the flank will produce nothing but a faint flicker of the under- 

 lying muscles, the same touch on the same part of a frog poisoned 

 with strychnia will produce violent lasting tetanic contractions of 

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

 seen travel along motor nerves without any great expenditure of 

 energy and probably without increasing that expenditure as they 

 proceed ; and the same is apparently 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 term " 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 and 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 case 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 movement, 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 to the afferent 

 impulses. At the same time we are able to recognize in most 

 reflex actions a certain relation between the strength of the 

 stimulus, that is to say the magnitude of the afferent impulses, 

 and the extent of the movement, that is to say 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 



10 



