CHAP, i.] THE SPINAL COED. 703 



descending from the brain. The movements of reflex origin, 

 in all their features except their exciting cause, appear identi- 

 cal with voluntary movements; the two can only be distin- 

 guished from each other by a knowledge of the exciting cause.. 

 And it seems unreasonable to suppose that the spinal cord 

 should possess two sets of mechanisms in all respects identical 

 save that the one is discharged by volitional impulses from 

 the brain and the other by afferent impulses from afferent 

 nerves. We are led therefore to the conclusion that in a reflex 

 action two kinds of afferent impulses are concerned: the ordi- 

 nary afferent impulses which discharge the nervous mechanism 

 within the cord and so provoke the movement, and the afferent 

 impulses which connect that nervous mechanism with the mus- 

 cles about to be called into play, and which take part in the 

 coordination of the movement provoked. The nature of these 

 latter afferent impulses is at present obscure; but if we admit, 

 as we seem compelled to do, that the character of a reflex 

 action is determined by them as well as by the afferent im- 

 pulses which actually discharge the mechanism, the way is 

 opened for an explanation of the fact that when, as in the case 

 of the frog in question, the usual set of muscles cannot be em- 

 ployed by the nervous mechanism, recourse is had to another set. 



463. Lastly, the characters of a reflex movement are, as 

 we need hardly say, dependent on the intrinsic condition of the 

 cord. The action of strychnia just alluded to is an instance 

 of an augmentation of reflex action. Conversely, by various 

 influences of a depressing character, as by various anaesthetics 

 or other poisons, reflex action may be lessened or prevented. 

 So also, various diseases may so affect the spinal cord as to 

 produce on the one hand increased reflex excitability so that a 

 mere touch may produce a violent movement, and on the other 

 hand diminished reflex excitability so that it becomes difficult 

 or impossible to call forth a reflex action. 



In the mammal the study of reflex action is rendered some- 

 what difficult by the effects of shock to which we referred 

 above. For days even after division of the spinal cord the 

 parts of the body supplied by nerves springing from the cord 

 below the section may exhibit very feeble reactions only. In 

 the dog, for instance, after division of the spinal cord in the 

 lower thoracic region, the hind limbs hang flaccid and motion- 

 less, and pinching the hind foot evokes as a response either 

 slight irregular movements or none at all. Indeed were our 

 observations limited to this period we might infer that the 

 reflex actions of the spinal cord in the mammal were but feeble 

 and insignificant. If however the animal be kept alive for a 

 longer period, for weeks or better still for months, though no 

 union or regeneration of the spinal cord takes place, reflex 

 movements of a powerful, varied and complex character mani- 



