CHAP, i.] THE SPINAL CORD. 905 



body ; a very feeble stimulus applied to the skin of these regions 

 promptly gives rise to extensive and yet coordinate movements. 

 Indeed the more the matter is studied, the stronger is the 

 evidence that the reflex movements carried out by isolated 

 portions of the spinal cord of the mammal are hardly less definite, 

 complete and purposeful, than those witnessed in the frog. It i> 

 worthy of attention, as bearing out the remarks made above on 

 the great differentiation of the central nervous system in the 

 higher animals, that the reflex phenomena in mammals vary very 

 much not only in different species but also in different individuals 

 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. It has been asserted that the 

 isolated spinal cord of the rabbit, which has been the subject of so 

 many experiments, is, as compared with that of the dog and 

 many other mammals, singularly deficient in the power of carrying 

 out complex reflex movements. 



In studying reflex actions in man we are met with the 

 difficulty that we never have to deal with a portion of the spinal 

 cord separated from the rest of the central nervous system under 

 the favourable circumstances of experimental investigation. In 

 man, we must be content to examine reflex actions either while 

 the whole nervous system is intact, or when a portion of the cord 

 has been wholly or partially separated by some more or less diffuse 

 disease or by some accident involving more or less crushing of the 

 nervous structures. Hence, the caution already given, as to 

 drawing inferences concerning man from the results of experi- 

 ments on animals, acquires still greater force. 



584. Confining ourselves at first to the results of experi- 

 ments on animals we may say that in both 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 manoeuvre. And in all reflex movements, both simple 

 and complex, we can recognize certain determining influences 

 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 characters 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 muscles; 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 



