ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. 



673 



limal life. Strychnine is only poisonous to the nerve-cell, not to 

 plant-cell. 



Every time that intellectual action is suppressed then are the 

 jflexes more manifest. A person asleep has more energetic reflex 

 itions than a person awake. In somnambulism the action of the 

 r ill is nearly suppressed, while the reflex excitability of the cord is 

 jnormously increased. 



On the other hand, a person by exercising a strong will can 

 irrest certain reflexes. Thus, the conjunctival reflex can be pre- 

 rented by the will of a courageous person. Up to a certain point a 

 >rson is able to resist sneezing or coughing, which are certainly 

 typical reflex movements. 



SWIFTNESS OF EEFLEX ACTIONS. Helmholtz succeeded in meas- 

 ing by the graphic method the switfness of the spinal actions. By 

 im it was ascertained that the excitation travels in the spinal cord 

 it the rate of about twenty-four feet per second. 



LAWS OF EEFLEX ACTIONS. They are the law of localization 

 ind that of irradiation. One other accessory law will be added : the 

 iw of co-ordination. 



Law of Localization. If any sensitive region be excited, the 

 reflex movement which will be produced will bear upon the 

 mscles near the sensitive region excited. 



Thus, if the foot of a frog be very lightly touched, the muscles 

 that foot will respond reflexly. If the conjunctiva be touched, 

 le reflex movement will be in the orbicular muscles. 



Law of Irradiation. When an excitation has produced a reflex 

 lovement in the muscles of one side by a first degree of irradiation, 

 there will be reflex movements in the corresponding muscles of the 

 >pposite side. Cutaneous constriction by cold applied to the right 

 id determines constriction by the vasomotors of the left hand as 

 , These are examples of the type known as transverse irradiation. 

 If the excitation be more intense, the movement is spread into 

 the muscles situated above and below the point of excitation. This 

 ^presents the longitudinal irradiation. 



Law of Co-ordination. The law of co-ordination or adaptation 

 )f the reflex actions in decapitated animals is very striking. If a 

 Irop of acetic acid be placed upon the back of a decapitated frog the 

 mimal will make such movements with the feet that it seems to 

 rant to free itself from the substance which irritates it. They are 

 lot blind movements, but such as seem to be adapted to an end and 

 co-ordinated. 



