704 REFLEX ACTIONS. [Book in. 



fest 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 coordinate movements. 

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

 dence that the reflex movements carried out by isolated por- 

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

 complete and purposeful, than those witnessed in the frog. 

 And the main points on which we have dwelt above in relation 

 to the frog hold good for the mammal. It is worthy of atten- 

 tion, 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 indi- 

 viduals and in the same individual under different circum- 

 stances. 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 circum- 

 stances, 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 com- 

 pared with that of the dog and many other mammals, singu- 

 larly deficient in the power of carrying out complex reflex 

 movements. 



§ 464. When we come to study the reflex actions of man 

 we should at first perhaps be inclined to infer that, since in 

 him the spinal cord is so largely used as the instrument of the 

 brain, the independent reflex actions of the cord, at least such 

 as affect skeletal muscles, are in him of much less importance 

 than they appear to be in animals ; and experience seems to 

 support this view. But it must be remembered that in his 

 case, as we have already stated (§ 458), we lack the guidance 

 of experimental results ; we are obliged to trust to the entan- 

 gled phenomena of disease or to a study of the behaviour of 

 the cord while it is still a part of an intact nervous system ; 

 and each of these methods presents difficulties of its own. The 

 movements, which in the intact human body we can recognize 

 as indubitable reflex actions, are as a rule simple and unimpor- 

 tant. They are, in by far the greater number of instances, 

 occasioned by stimulation of the skin or of the mucous mem- 

 brane, for the most part involve a few muscles only, and rarely 

 indicate any very complex coordination. The flexion, followed 

 by extension, of the leg which is called forth by tickling the 

 sole of the foot, may perhaps be regarded as the type of these 

 movements. A very common form of reflex action is that in 

 which a muscle or group of muscles is thrown into contraction 

 by stimulation of the overlying or neighbouring skin, as when 

 the abdominal muscles contract upon stroking the skin of the 



