THE REFLEX ACTIONS OF THE SPINAL CORD. 567 



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 cir- 

 cumstances, such as whether food has been recently taken or not. 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 favorable circumstances of experi- 

 mental 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 struc- 

 tures. Hence, the caution already given, as to drawing inferences concern- 

 ing man from the results of experiments on animals, acquires still greater 

 force. 



497. Confining ourselves at first to the results of experiments 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 gen- 

 erated 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 or other stimulation of the skin than by even 

 strong induction-shocks applied directly to a nerve-trunk. If in a brainless 

 frog, the area of skin supplied by one of the dorsal cutaneous nerves be sepa- 

 rated by section from the rest of the skin of the back, the nerve being left 

 attached to the piece of skin and carefully protected from injury, it will be 

 found that slight stimuli applied to the surface of the piece of skin easily 

 evoke reflex actions, whereas the trunk of the nerve may be stimulated with 

 even strong currents without producing anything more than irregular move- 

 ments. In ordinary mechanical and chemical stimulation of the skin it is 

 not a single impulse but a series of impulses which passes upward along the 

 sensory nerve, the changes in which may be compared to the changes in a 

 motor nerve during tetanus. In every reflex action, in fact, the central 

 mechanism may be looked upon as being thrown into activity through a 

 summation of the afferent impulses reaching it. Hence while a reflex action 

 is readily called forth by even feeble induction-shocks applied to the skin 

 if they be repeated sufficiently rapidly, a solitary induction-shock is inef- 

 fectual unless it be strong enough to cause in the skin or nerves changes 

 of an electrolytic nature sufficient to give rise of themselves to a series of 

 impulses. 



498. When a muscle is thrown into contraction in a reflex action, the 

 pitch of the sound which it gives forth does not vary with the stimulus, but 



