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TEXT-BOOK OF PHYSIOLOGY 



follow each other with a rapidity sufficient to give rise to a summation of 

 effects in the nerve-centers which will then be followed by a muscle response. 

 It is highly probably that the chemic stimulation gives rise to a similar sum- 

 mation of effects. 



The period of time thus obtained is distributed over the entire mechan- 

 ism. The true reflex time, however i.e., the time occupied in the passage 

 of the nerve impulses through 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 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. 



olf.n. 



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. 



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 skeletal muscles. Under its influence 

 the usual coordinated reflexes disappear and are succeeded by in- 

 coordinated 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 radiated in all direc- 

 tions. Absolute repose of the animal and the exclusion of all external 

 stimuli greatly diminish the tendency to the occurrence of spasms. 

 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 



-tned.ob. 



FIG. 232. DIAGRAM OF THE BRAIN OF 

 THE FROG. olj. n. olfactory nerves \olj.l 

 olfactory lobes ; c. h. cerebral hemispheres ; 

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

 c. cerebellum; med. ob. medulla oblon- 

 gata; IV. v. fourth ventricle. 



