520 



THE NERVOUS SYSTEM 



and may involve many complicated nervous activities and even psychological 

 processes. Where the response is approximately immediate, the reaction is 

 known as a reflex. 



A reflex arc, reduced to its simplest terms, consists of the following ele- 

 ments: a, a sensory surface; b, an afferent neurone; c, an efferent neurone; d, 



FIG. 364. Schematic Sketch of a Reflex Arc. A , With two neurones, an afferent and an efferent; 

 B, with three neurones, an afferent, efferent, and a connecting or intracentral neurone. 



a muscle or gland. The simplest form of reflex arc is schematically shown 



in figures 364 and 365. 



The gap between the termination of the afferent neurone and the dendron 



of the efferent neurone shown in figure 364 is called a synapsis. The reflex arc is 



probably seldom as simple as that shown in figure 365, where only two neurones 



are involved. More often, three or more 

 neurones take part, as shown in figures 364 B, 

 and 366. 



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 in- 

 directly connected with one another by 

 intracentral neurones, figure 361, the possi- 

 bility of increasing the number of efferent 

 limbs of the reflex arc can be readily under- 

 stood. 



A physiological reaction in a tissue pro- 

 duced by efferent nerve impulses which have 

 been discharged from a nerve center under 

 the stimulus of a sensory or afferent nerve 

 impulse, is called a reflex act. Where the 

 nervous apparatus involved is of the type 

 represented in figures 364 and 365, the activ- 

 ity is called a simple reflex. Most reflexes 



FIG. 365. Sjiowing ^ the Arrangemen t 



of a Simple Reflex Mechanism Composed 

 of a Motor and Sensory Neurone, sg, 

 Posterior spinal ganglion; s and sth, sen- 

 sor^ root; m, motor-nerve cell; mw, 

 motor root. (Kolliker.) 



