460 TEXT-BOOK OF PHYSIOLOGY. 



muscle response. It is highly probable that the chemic stimulation 

 gives rise to a similar summation of effects. 



The period of time thus obtained is distributed over the entire 

 mechanism. The true reflex time, however, i. e., the time occupied 

 in the passage of the nerve impulses across the spinal mechanism, 

 is shorter and is obtained by subtracting from the whole period 

 the time occupied by the passage of the impulses through the afferent 

 and efferent nerves as well as the latent period of muscle contraction. 

 This corrected period, the true reflex time, has been found to be 

 twelve times longer than the time occupied by the passage of the 

 nerve impulse through the nerves, including the latent period of the 

 muscle. 



The reflex irritability is increased by : 



1. Separation of the Brain jrom the Cord. This is at once followed 



by an increase in reflex irritability, and is taken as evidence that 

 the brain normally exerts an inhibitory influence over the reflex 

 centers of the cord. The same increase is observed upon hemi- 

 section of the cord, though the increase is limited to the same 

 side. 



2. The Administration of Strychnin. Even in small doses this alka- 



loid increases the irritability to such an extent that a minimal 

 stimulus 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 im- 

 pulses 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 radiated 

 in all directions. Absolute repose of the animal and the exclu- 

 sion of all external stimuli greatly diminish the tendency to 

 the occurrence of spasms. 



3. Degeneration of the Pyramidal Tracts. In primary lateral scle- 



rosis, a pathologic condition characterized primarily by a degen- 

 eration of the terminal filaments of the pyramidal tract fibers, 

 the reflex activity of the cord becomes exalted. As the disease 

 progresses the irritability increases to such an extent that violent 

 spasmodic contractions of the arms and legs arise when the skin 

 or tendons are mechanically stimulated. The explanation 

 offered is practically the same as in division of the cord: viz., 

 withdrawal of the inhibitory and controlling influence of the 

 brain. 



The reflex excitability may be decreased by: 



i. Stimulation of Certain Regions of the Brain. It was discovered 

 by Setchenow that when the frog brain is divided just anterior 



