THE SPINAL CORD AND REFLEX 



not tending to produce any definite result, and affecting either 

 a limited region or all the muscles of the body. 



In higher animals similar phenomena may be observed. If 

 a rabbit's spinal cord be divided at the bottom of the neck the 

 animal is at first thrown into a flaccid limp condition like the 

 frog, but it soon recovers. Voluntary movements in muscles 

 supplied from the spinal cord behind the section are never seen 

 again ; but 011 pinching the hind foot it is forcibly withdrawn. 

 Men, whose spinal cord has been divided by stabs or disease 

 below the level of the fifth cervical spinal roots (above which 

 the fibres of the phrenic nerve, which are necessary for breath- 

 ing, pass out) , sometimes live for a time, but can no longer move 

 their legs by any effort of the will, nor do they feel touches, 

 pinches, or hot things applied to them ; if, however, the soles 

 of the feet be tickled the legs are thrown into vigorous move- 

 ment. As a rule, however, orderly reflexes are less marked 

 and less numerous in the higher animals ; in them the organ- 

 ization is less machine-like, the spinal cord being more the 

 servant of the larger brain, and less capable of working with- 

 out directions. Such animals, when intact, can to a greater 

 extent control the muscular responses which shall be made to 

 stimuli under various conditions; they have less automatic 

 protection in the ordinary risks of life, but a greater range of 

 possible protection. The human spinal cord, controlled by 

 the brain, can adapt the reactions of the Body, with great 

 nicety, to a vast variety of conditions ; the frog's cord by itself 

 does this for a smaller number of possible emergencies without 

 troubling at all such brain as the animal has, but is less com- 

 pletely under the control of the higher centres for adaptation 

 to other and more complex conditions. The difference being, 

 however, but one of degree and not of kind, it is best to 

 approach the study of the reflex actions of the human spinal 

 cord through an examination of tjiose exhibited by the frog. 



The Ordinary Reflex Movements of a Decapitated 

 Prog. F or the occurrence of these the following parts must be 

 intact : (a) the end organs of sensory nerve-fibres ; (b) afferent 

 fibres from these to the cord; (c) efferent fibres from the 

 cord to the muscles; (d) the part of the spinal cord between 

 the afferent and efferent fibres ; (e) the muscles concerned in 

 the movement. If the decapitated animal be suspended ver- 

 tically after the shock of the operation is over, it makes a few 

 attempts to hold its hind legs in their usual flexed position ; 



