THE SPINAL CORD. 



olf.n. 



The same increase is observed upon hemisection of the cord, though 

 the increase is limited to the same side. 



The Toxic Action of Drugs. Many drugs increase the irritability of the 

 spinal cord, though the most efficient is strychnin. This drug, even in 

 small doses, increases the irritability to such an extent that a minimal stim- 

 ulus* is sufficient to call forth spasmodic contractions of all the skeletal 

 muscles. Under its influence the usual coordinated reflexes disappear 

 and are succeeded by incoordinated reflexes. The explanation of this 

 fact is believed to be a diminution in 

 the resistance offered by the cord to 

 the passage of the afferent impulses 

 rather than to a direct stimulation of 

 the efferent cells. So much is this 

 resistance decreased that the nerve 

 impulses instead of being confined 

 to their accustomed paths, are ra- 

 diated in all directions. Absolute 

 repose of the animal and the ex- 

 clusion of all external stimuli greatly 

 diminish the tendency to the occur- 

 rence of spasms. 



Degeneration 0} the Pyramidal Tracts. 

 In primary lateral sclerosis, a 

 pathologic condition characterized 

 primarily by a degeneration of the 

 terminal filaments of the pyramidal 

 tract fibers, the reflex activity of the 

 cord becomes exalted. As the dis- 

 ease progresses the irritability in- 

 creases to such an extent that vio- 

 lent spasmodic contractions of the 

 arms and legs arise when the skin 

 or tendons are mechanically stimu- FlG . 235. DIAGRAM OF THE BRAIN OF 



lated. The explanation offered is THE FROG. olf. n. olfactory nerves ;olf.l. 



olfactory lobes ; c. h. cerebral hemispheres ; 

 op. thl. optic thalamus; op. I. optic lobes; 

 c. cerebellum; med. ob. medulla oblon- 



practically the same as in division 



of the cord: viz., withdrawal of the 



inhibitor and controlling influence g ata ; IV - v - fourth ventricle. 



of the brain. 



The reflex excitability may be decreased by: 



Stimulation of Certain Regions oj the Brain. It was discovered by Setche- 

 now that when the frog brain is divided just anterior to the optic lobes 

 (Fig. 235) and the reflex time subsequently determined according to the 

 method of Tiirck, the time can be considerably lengthened by stimula- 

 tion of the optic lobes. This is readily accomplished by placing small 

 crystals of sodium chlorid on the optic lobes. It was concluded from 

 this fact that these lobes contain centers which exert an inhibitor in- 

 fluence over centers in the spinal cord through descending nerve-fibers. 

 This conclusion is strengthened by the fact that division of the brain 

 just behind the optic lobes causes a temporary inhibition of the 



