552 



THE NERVOUS SYSTEM 



development and by its comparative anatomy. The pairs of nerves are 

 indicative of the component segments of the cord. The tracts of the cord are 

 in a sense connectives from segment to segment, connecting the cells of 



FIG. 366. 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 intra- 

 central neurone. 



both adjacent and of widely separated segments. The function of the cord 



is comprised in the function of the segments and in the function of the tracts. 



From a physiological point of view, it may almost be considered as an 



axiom that before a nerve cell can send out a nerve impulse it must first 



receive a stimulus of some kind. 



mw. 



P'iG. 367. Showing the Arrange- 

 ment of a Simple Reflex Mechanism 

 Composed of a Motor and Sensory 

 Neurone, sg, Posterior spinal gan- 

 glion; s and sth, sensory root; m, 

 motor-nerve cell; mw, motor root. 

 (Kolliker.) 



This stimulus usually consists of an afferent 

 impulse from the periphery. Its effect 

 upon the receiving cell may be insufficient 

 to cause any response, or the response may 

 be delayed for a long period and may in- 

 volve many complicated nervous activities 

 and even psychological processes. Where 

 the peripheral response is approximately 

 immediate, the reaction is known as a 

 reflex. 



A reflex arc, reduced to its simplest 

 terms, consists of the following anatomical 

 elements: a, a sensory surface; ft, an affer- 

 ent neurone; c, an efferent neurone; d, a 

 muscle or gland. The simplest form of 

 reflex arc is schematically shown in figures 

 366 and 367. 



The interlocking of fibers between 

 the termination of the afferent neurone 

 and the dendrite of the efferent neurone 

 shown in figure 366 is called a synapse. 

 The reflex arc is probably seldom as 

 simple as that shown in figure 367, 

 where only two neurones are involved. 



