REFLEX ACTION OF THE SPINAL CORD. 413 



limbs, therefore, have been directly paralyzed, the nerves of the same 

 parts have retained their irritability. 



2. If a frog be poisoned with woorara by simply placing the 

 poison under the skin, no reflex action of the spinal cord can be 

 demonstrated after death. We have already shown, from experi- 

 ments detailed in Chapter II., that this substance destroys the irrita- 

 bility of the motor nerves, without affecting that of the muscles. In 

 the above instance, therefore, where the reflex action is abolished, its 

 loss may be owing to a paralysis of both motor and sensitive fila- 

 ments, or to that of the motor filaments alone. The following experi- 

 ment, however, shows that the motor filaments are the only ones 

 affected. If a frog be prepared as in Fig. 137, and poisoned by the 

 introduction of woorara at /, when the limb d is irritated its own 

 muscles react, while no movement takes place in a, b, or c ; but if 

 the irritation be applied to a, b, or c, reflex movements are imme- 

 diately produced in d. In the poisoned limbs, therefore, while the 

 motor nerves have been paralyzed, the sensitive filaments have retained 

 their irritability. 



3. If a frog be poisoned with strychnine, introduced underneath 

 the skin in sufficient quantity, death takes place after general con- 

 vulsions, which are due, as we have seen above, to an unnatural 

 excitability of the reflex action. This is followed, however, by a 

 paralysis of sensibility, so that after death no reflex movements 

 can be produced by irritating the skin or even the posterior roots 

 of the spinal nerves. But if the anterior roots, or the motor nerves 

 themselves be galvanized, contractions immediately take place in 

 the corresponding muscles. In this case, therefore, the sensitive fila- 

 ments have been paralyzed, while the motor filaments and the muscles 

 have retained their irritability. 



AVe now come to investigate the reflex action of the spinal cord, 

 as it takes place in a healthy condition during life. This action 

 readily escapes notice, unless our attention be particularly directed 

 to it, because the sensations which we are constantly receiving, and 

 the many voluntary movements which are continually executed, 

 serve naturally to mask those nervous phenomena which take place 

 without our immediate knowledge, and over which we exert no 

 voluntary control. Such phenomena, however, do constantly take 

 place, and are of extreme physiological importance. If the surface 

 of the skin, for example, be at any time unexpectedly brought in 

 contact with a heated body, the injured part is often withdrawn by 

 a rapid and convulsive movement, long before we feel the pain, or 



