586 REFLEX ACTIONS. [BOOK in. 



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

 lower dorsal region, the hind limbs hang flaccid and motionless, 

 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 manifest themselves in the hind limbs and 

 hinder parts of the body ; a very feeble stimulus applied to the 

 skin of these regions promptly gives rise to extensive and yet co- 

 ordinate movements. Compared with the reflex actions of the 

 frog, the movements carried out by the lower portion of the spinal 

 cord of the mammal while they are more energetic have hitherto 

 been regarded as being less definite and complete and less 

 purposeful ; but it would be dangerous to insist on this, for recent 

 experience tends to shew that, in the case of most mammals, the 

 powers of the spinal cord have been unduly underrated. It is 

 worthy of attention that the reflex phenomena in mammals vary 

 very much not only in different species but also in different in- 

 dividuals and in the same individual under different circumstances. 

 Race, age, and previous training, seem to have a marked effect in 

 determining the extent and character of the reflex actions which 

 the spinal cord is capable of carrying out ; and these seem also to 

 be largely influenced by passing circumstances, such as whether food 

 has been recently taken or no. And it is asserted that the spinal 

 cord of the rabbit, which has been the subject of so many experi- 

 ments, is, as compared with that of the dog and many other 

 mammals, singularly deficient in the power of carrying out complex 

 reflex movements. 



Both in the cold-blooded and warm-blooded animals the salient 

 feature of ordinary reflex actions is their purposeful character, 

 though every variety of movement may be witnessed, from a simple 

 spasm to a most complex manoauvre ; and in all reflex movements, 

 both simple and complex, we can recognise certain determining 

 causes, the influences of which more or less directly contribute 

 to the shaping of this purposeful character. 



Thus the features of any movement taking place as part of a 

 reflex action are in part determined by the nature 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 *nuscles ; 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 the skin than by 

 even a strong single induction-shock applied directly to a nerve- 



