FUNCTIONS OF THK SPINAL CORD. 791 



the motor ganglia of the cord, and so produce violent convulsive move- 

 ments; or the receptivity of the cord may be obtunded through disease 

 or through the action of various poisons, and the most violent stimulus 

 now fail to evoke any reflex action. As an example of the first condition 

 strychnine furnishes a most striking illustration. 



If a frog be poisoned with strychnine and the cerebrum removed, a 

 degree of stimulation which otherwise would produce but a feeble or 

 perhaps even no reflex action now produces tetanic contractions. Such 

 a result indicates that in the spinal cord every sensible fibre is in direct 

 communication through the gray substance with every motor fibre. 



In the frog at breeding seasons the spinal cord is in a physiological 

 state of overexcitation ; the frog is found at this time clinging obstinately 

 to pieces of bark or stone; just as it does to the body of the female in 

 the act of copulation. Such a condition may be produced by gentle 

 stimulations of the skin of the sternum and of the thumb of the frog, in 

 which an increase of sensibility exists. Here the result is to be attributed 

 not only to the increased receptivity of the spinal cord, but also to the 

 increased sensitiveness of the receptive surfaces. 



In the normal condition Of animals, whether mammals or cold- 

 blooded animals, in whom the brain has been separated from the spinal 

 cord, reflex action only takes place through the application of irritants 

 of a certain intensity and a certain duration. 



Single electric shocks as a rule produce no result, but if repeated 

 sufficiently often produce a reflex action ; such single impulses are con- 

 ducted to the spinal cord and there become added to each other by what 

 is known as a process of summation until a maximum result is attained. 

 If, then, the number of stimulations per second be increased or the de- 

 gree of stimulation be made more severe no further increase in the reflex 

 action is possible. 



Pfliiger has formulated the following laws of reflex action : — 



1. The reflex movement occurs on the same side on which the sen- 

 sory nerve is stimulated, while only those muscles contract whose nerves 

 arise from the same segment of the spinal cord. 



2. If the reflex occur on the other side only the corresponding 

 muscles contract. 



3. If the contractions be unequal upon two sides, then the most 

 vigorous contractions always occur on the side which is -stimulated. 



4. If the reflex excitement extend to the other motor nerves, those 

 nerves are also affected which lie in the direction of the medulla oblon- 

 gata. 



5. All the muscles of the body may be thrown into contraction. 

 (Landois.) 



In the human body are found mechanisms which may inhibit or 



