548 



TEXT-BOOK OF PHYSIOLOGY 



olf.n.. 



...cA 



mechanism is shorter and is obtained by subtracting from the whole 

 period the time occupied by the passage of the impulses through the affer- 

 ent 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 total reflex time, that is, the time 

 elapsing between the application of the 

 stimulus and the response of an organ 

 may - be shortened by influences which 

 heighten the irritability of any one or 

 more portions of the reflex arc: e.g. 



The irritability may be heightened 

 by a variety of causes, e. g. y 



1. Separation of the Brain from the Cord. 



This is at once followed by an in- 

 crease in reflex irritability, and is 

 taken as evidence that the brain is 

 normally exerting an inhibitor in- 

 fluence over the reflex centers of the 

 cord. The same increase is ob- 

 served upon hemisection of the cord, 

 though the increase is limited to the 

 same side. 



2. 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 

 stimulus is sufficient to call forth 

 spasmodic contractions of all the 



ispheres; op. M. optic thalamua; op. l. skeletal muscles. Under its influ- 



optic lobes; c. cerebellum; med. ob ence the usual coordinated re- 



medulla oblongata; IV. v. fourth ,, , , , 



ventricle. flexes disappear and are succeeded 



by incoordinated reflexes. The 



explanation of this fact is believed to be a diminution in the resist- 

 ance 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 radiated in all directions. Absolute 

 repose of the animal and the exclusion of all external stimuli greatly 

 diminish the tendency to the occurrence of spasms. 



3. Degeneration of 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 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 stimu- 

 lated. The explanation offered is practically the same as in division 



-. med. ob. 



FIG. 227. DIAGRAM OF THE BRAIN 



