592 



THE NERVOUS SYSTEM 



More often, three or more neurones take part, as shown in figures 366 B, 

 and 368. 



The neurone connecting the afferent neurone with the efferent neurone 

 belongs to the class of intracentral or connecting neurones. Since all parts 

 of the cord, in fact of the entire cerebro-spinal axis, are directly or indirectly 

 associated with one another by central neurones, figure 363, the possibility 

 of increasing the number of efferent limbs of the reflex arc can be readily 

 understood. 



An involuntary physiological reaction in a tissue produced by efferent 

 nerve impulses which have been discharged from a nerve center under the 



stimulus of a sensory or afferent nerve im- 

 pulse, is called a reflex act. Where the 

 nervous apparatus involved is of the type 

 represented in figures 366 and 367, the 

 activity is called a simple reflex. Most re- 

 flexes are more complex in character. The 

 afferent nerve impulse passes through more 

 than one simple channel in the cord, so that 

 in response to a sensory stimulus a series of 

 coordinated acts occurs in what may be 

 called a complex reflex. 



The transmission of impulses within the 

 cord occurs over the pathways of least re- 

 sistance. Increasing a number of synapses, 

 or the number of neurone links in the chain 

 of conduction, increases the resistance so that 



reflexes will occur most readily, other conditions being equal, where the small- 

 est number of neurones is involved, i.e., in the same segment of the cord in 

 which the sensory impulse enters, or in immediately adjacent segments. In 

 addition to the number of synapses in the reflex arc, certain strictly physio- 

 logical factors are of importance in determining reflex reaction; e.g., the in- 

 tensity of the exciting stimulus; the quality of the stimulus; the rapidity of 

 the recurrence of the stimulus; and the duration of its application. Thus, 

 a strong stimulus will bring about a reflex reaction sooner than a weak stimu- 

 lus of the same kind. A single weak stimulus which will cause no reflex may 

 do so if often enough and rapidly enough repeated, the phenomenon of 

 summation of stimuli. 



A reflex act once started may result in efferent impulses which continue 

 for some time after the exciting cause has been removed. The same phe- 

 nomenon is observed where groups of nerve cells are stimulated directly. It 

 has been found, by observing electrical changes in nerve fibers by means of 

 the capillary electrometer, that when their cells of origin are stimulated they 

 discharge impulses in a rhythmical manner. 



FIG. 368. Showing the Arrange- 

 ment of the reflex mechanism, with 

 a neurone intercalated between the 

 sensory and motor neurones. 



