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THE CENTRAL NERVOUS SYSTEM 



branching of the end of the axoii (Fig. 207), it may really consist of a 

 sort of membrane the synaptic membrane. It permits the nerve im- 

 pulse to pass in one direction only, from synapsis to cell. Of what this 

 membrane may be composed, we do not know, but there can be no 

 doubt as to its great functional importance in connection with the in- 

 tegration of the central nervous system; for example, failure of an im- 

 pulse to pass between two neurons may be due to retraction of the 

 synaptic membrane from the cell, or to alteration in its permeability to- 

 wards the nerve impulse, perhaps as a consequence of changes in surface 



Fig. 207. Arborization of collaterals from the posterior root fibers around the cells of the 

 posterior horn. A, ascending fiber in posterior columns; B, collaterals; C, cells of posterior hor/i; 

 R, synapsis. (From Ramon y Cajal.) 



tension. Similar changes may also be brought about by the action of 

 electrolytes or by chloroform, strychnine, and other drugs. As we shall 

 see when we come to study the reflexes of the higher animals, there can 

 be little doubt that it is in the synaptic membrane that many of the 

 peculiarities reside which characterize conduction in a reflex arc as 

 compared with that in a nerve trunk. The phenomena of summation, 

 of reciprocal inhibition, of facilitation, etc., are undoubtedly depend- 

 ent upon such alterations. The synapsis is also almost certainly the 

 seat of fatigue in the central nervous system, and it is possibly the 

 structure whose physiologic activity becomes upset in surgical shock. 



