196 CENTRAL NERVOUS SYSTEM 



nerve structures. Nerves do pass out indiscriminately, but 

 are grouped together into anatomical structures forming the 

 cranial, spinal, and sympathetic systems. An intelligent 

 appreciation of the physiology of the central nervous system 

 must rest upon a knowledge of its structure. This, however, 

 is excessively intricate and but incompletely known. In its 

 simplest aspect it may be regarded as based upon reflex arcs, 

 made up of afferent and efferent neurons. These with perhaps 

 one or more central cells or tract cells form a chain of nerve 

 cells which conveys a disturbance arising in a peripheral sen- 

 sory end organ to the central nervous system and back to a 

 reacting organ (muscle or gland). This event, if it is involun- 

 tary, is a reflex act. 



Reflexes. Reflexes fall into three groups: (1) Simple reflexes, 

 in which a single muscle is effected, for example, the winking 

 reflex: (2) coordinated reflexes, in which a number of muscles 

 react in time and extent so as to produce an orderly and useful 

 movement; (3) convulsive reflexes, in which many muscles 

 contract simultaneously, producing disorderly and useless 

 movements. The latter may be produced by a very intense 

 stimulation or by heightening the irritability of the central 

 nervous system, as, for instance, by the injection of strychnine 

 or tetanus toxin. The mechanism underlying a simple reflex 

 may be conceived to be that of a simple reflex arc, consisting 

 of a receiving structure or receptor, a conducting structure or 

 conductor, and a reacting structure or effector. The greater com- 

 plexity of the coordinated reflex implies a corresponding com- 

 plexity of reflex arc, so that an impulse reaching the cord along 

 a single afferent fiber has presented to it the choice of many 

 potential routes. The precise route or routes followed depends 

 upon the varying resistances offered by the synapses. A nerve 

 impulse comes into the cord along a path traversed only by 

 itself and others of like kind, but passes to the effector organ 

 along a path used by many different impulses coming from widely 

 separated areas of the body. The latter is, therefore, desig- 

 nated as the final common path. If two impulses, destined 

 when acting alone, to produce antagonistic effects, should 

 simultaneously enter upon the same final common path, then 

 onlv one becomes effective. 



