Responses of Higher Animals: The Nervous System - 465 



opposite side of the body. Injury to the spinal 

 cord, therefore, results in serious motor im- 

 pairments, in addition to the sensory losses 

 mentioned previously. If only one side of the 

 cord is crushed, all the muscles of that side, 

 in segments below the level of the damage, 

 are divorced from the higher centers and can- 

 not be utilized for voluntary movements. 

 The paralysis is not complete, however, since 

 below the level of the injury, the intraseg- 

 mental and intersegmental reflex arcs are still 

 intact, permitting the fulfillment of local 

 reflexes after the animal has recovered from 

 a "temporary shock," which lasts for some 

 hours after the injury. 



The Autonomic Nerves: Visceral Reflexes. 

 The responses of the internal organs, in the 

 digestive, respiratory, urogenital, and circu- 

 latory systems, are performed mainly through 

 the agency of visceral muscle and glands, or, 

 in the case of the heart, by cardiac muscle. 

 These visceral effectors differ from skeletal 

 muscles, in that each receives a double set of 

 motor fibers from the central nervous system. 

 One set of fibers comes to the organ by way 



of the sympathetic nerves, and the other 

 fibers by way of the parasympathetic nerves. 

 Moreover, the sympathetic and parasym- 

 pathetic fibers always antagonize each other 

 as to their action upon the visceral organ. 

 Thus if one set of fibers augments activity, 

 the other invariably depresses activity in the 

 organ, as may be seen in Table 25-2. 



By means of such a mutually antagonistic 

 action, the sympathetic and parasympathetic 

 nerves exert a joint control in all visceral 

 reflexes; and thus it is convenient to desig- 

 nate the sympathetic and parasympathetic 

 nerves collectively as the autonomic nervous 

 system (Fig. 25-16). Actually the autonomic 

 system is just a physiological subdivision of 

 the peripheral nervous system, and the auto- 

 nomic nerves, like other peripheral nerves, 

 depend upon the brain and spinal cord for 

 central connections in the completion of all 

 reflex arcs. Visceral reflexes, however, gen- 

 erally have their association centers in the 

 brain stem (medulla), and visceral reflexes 

 seldom reach the level of consciousness, or 

 voluntary control. Thus the rhythm of the 



Table 25-2— Action of the Autonomic Nerves 



