CHAPTER CIV 



SUMMARY OF THE ORGANIZATION OF THE MAMMALIAN NERV- 

 OUS SYSTEM ; SPINAL SHOCK 



The activity and organization of the nervous system is so complex 

 that it has been necessary to treat each aspect of it separately and with 

 little reference to its relation to the organization of the whole. We will 

 now consider briefly how the various parts of the nervous system are com- 

 pounded to form a unified machine for coordinating the activities of the 

 body. The evolutionary development of the nervous system has indi- 

 cated that two primitive types of nervous organization came into 

 existence at an early stage and persist in certain parts of the mammalian 

 body. The most primitive of these was the nerve net which was evolved 

 by the coelenterates and which persists in the mesenteric plexus of 

 man. The other is the segmental synaptic nervous system of the worms, 

 which finds its representative in the spinal reflex system of the verte- 

 brates. We have seen that these systems are not independent, but that 

 the spinal reflex mechanism exerts a controlling influence over the 

 peripheral nerve net through the autonomic nerves. Vertebrate evolution 

 has brought to perfection two additional functional systems which 

 modify and regulate the activity of the spinal reflexes, and through them, 

 to a certain extent, the organs controlled by the peripheral nerve net. 



These systems are (1) the mesencephalicospinal system for the con- 

 trol of postural tone, excited by the proprioceptive impulses which 

 arise in the muscles, joints, and from the labyrinth and (2) the cortico- 

 spinal system for the execution of voluntary movements initiated by im- 

 pulses received in large part by the distance receptors of the head and 

 conditioned by the previous associations and memories which the stimula- 

 tion of the distance receptors calls up. The corticospinal system not 

 only controls the activity of the spinal reflex mechanism through the 

 connections afforded by the pyramidal tracts, but to it the system for 

 regulation of postural tone is also subservient. The arrangement of the 

 different levels of nervous activity in their relation to the effectors of the 

 body is somewhat as shown in the diagram on page 964. 



In the intact organism we see the entire mechanism at work; in the 

 experimental animal and in the lesions of warfare and civil life we see 

 the activity of the residual parts of the mechanism which are left in- 

 tact after mutilation has freed them from control by the higher centers. 



963 



