588 REFLEX ACTIONS. [BOOK in. 



stimulus applied to an afferent nerve are limited to a few, those of 

 a strong stimulus may spread to many efferent nerves. Granting 

 that any particular afferent nerve is more especially associated with 

 certain efferent nerves than with any others, so that the reflex 

 impulses generated by afferent impulses entering the cord by the 

 former, pass with the least resistance down the latter, we must 

 evidently admit further that other efferent nerves are also, though 

 less directly, connected with the same afferent nerve, the passage 

 into the second efferent nerve meeting with an increased but not 

 insuperable resistance. When a frog is poisoned with strychnia, 

 a slight touch on any part of the skin may cause convulsions of the 

 whole body ; that is to say, the afferent impulses passing along any 

 single afferent nerve may give rise to the discharge of efferent im- 

 pulses along any or all of the efferent nerves. This proves that a 

 physiological if not an anatomical continuity obtains between 

 all the nerve-cells of the spinal cord which are concerned in reflex 

 action, that the nerve-cells with their processes form a functionally 

 continuous protoplasmic network. This network however we must 

 suppose to be marked out into tracts presenting greater or less re- 

 sistance to the progress of the impulses into which afferent impulses, 

 coming from this or that afferent nerve, are transformed on their 

 advent at the network; and accordingly the path of any series 

 of impulses in the network will be determined largely by the 

 energy of the afferent impulses. And the action of strychnia may 

 be in part explained by supposing that it reduces and equalises the 

 normal resistance of this network, so that even weak impulses 

 travel over all its tracts with great ease. 



Further, the movement forming part of a reflex action varies 

 in character, according to the particular area of the skin or 

 the locality of the body to which the stimulus is applied. 

 Pinching the folds of skin surrounding the anus of the frog 

 produces different effects from those witnessed when the flank 

 or toe is pinched; and, speaking generally, the stimulation 

 of a particular spot calls forth particular movements. In the 

 case of the simpler reflex movements, it appears to be a 

 general rule that a movement started by the stimulation of a 

 sensory surface or region on one side of the body, is developed 

 on the same side of the body, and if it spreads to the other side, 

 still remains most intense on the same side ; the movement on the 

 other side moreover is symmetrical with that on the same side. It 

 has been maintained that 'crossed' or diagonal reflex movements, 

 as where stimulation of one fore-foot leads to movements of the 

 opposite hind-limb, do not occur unless some portion of the medulla 

 oblongata be left attached to the spinal cord. Seeing that loco- 

 motion in four-footed animals is largely effected by diagonal move- 

 ments of the limbs, one would rather have expected to find the 

 spinal cord itself provided with mechanisms to assist in carrying 

 them out ; and indeed it is affirmed that in the case of cold- 



