412 REFLEX ACTIONS. 



Obs. XII. Having divided the medulla, make a tranverse 

 incision over the spine a little below the level of the fore limbs 

 (fig. 266, line c-d) cut through very carefully a vertebral arch 

 on each side of the middle line and remove the piece. With 

 a sharp-pointed scalpel, the spinal cord may be divided right 

 across. 



After the shock has passed away, it will be found that reflex 

 actions may be called forth in the fore limbs by stimulating 

 the skin of the fore limbs or of the fore part of the body, 

 without any movement whatever being produced in the hind 

 limbs ; and vice versa. By the operation, the body has be- 

 come divided into two segments, which, as far as all reflex 

 actions are concerned, are quite independent one of the other. 

 Sometimes, when the movements of one segment are very 

 violent, the other segment becomes displaced, the displace- 

 ment serves as a stimulus, and a reflex action is thereby indi- 

 rectly brought about. But this will not be confounded with 

 direct reflex actions, which can only be called forth by stimu- 

 lating the respective segments. 



Obs. XIII. In any of the above frogs which have shown 

 good reflex actions, destixty the spinal cord entirely by thrust- 

 ing a wire or blunt needle down the spinal canal. All reflex 

 actions will at once and for ever cease. 



Obs. XIV. The orderty and purposeful character of reflex 

 actions may be modified by the action of certain poisons, more 

 particularly by strychnia. 



Having divided the medulla in a frog, suspend the animal 

 as in Obs. III. and determine the readiness with which reflex 

 action is produced by mechanical stimulation. This m&y be 

 taken as a measure of the reflex excitability of the spinal cord 

 (the acid method being unsuitable in this case). 



Introduce into the back of the frog a w\> v or suW f a 

 grain of strj'chnia sulphate and determine again after a short 

 interval the effects of mechanical stimulation. Thc}^ will be 

 found to be increased, i. e., the reflex excitability has become 

 heightened. 



Now inject a larger quantity of the poison, and in a very 

 short time a very marked change becomes obvious. The 

 movement resulting from the stimulus is no longer a simple 

 movement, for instance, a simple withdrawal of the foot, but 

 a tetanic extension of the leg, which becomes more and more 

 violent and prolonged. 



Soon each application of the stimulus will give rise to a 

 prolonged tetanic movement which is no longer confined to 

 the linil:, or even to the side stimulated. The hind limbs are 

 forcibly extended, the fore limbs bent over the sternum, and 

 every muscle of the trunk is thrown into a state of prolonged 

 tetanic contraction. 



