PHYSIOLOGY OF THE NERVOUS 

 CENTERS. 



GENERAL CONSIDERATIONS. 



The central nervous organs are in general characterized by the follow- 

 ing properties : 



1. They contain nerve-cells, which, arranged in groups, are situated 

 either within the central organs of the nervous system or peripherally 

 in the course of the nerves. 



2. The nervous centers are capable of discharging reflexes, as, for 

 example, reflex movements, reflex secretion, reflex inhibition. 



3. The centers may be capable of automatic activity, that is, 

 apparently without external stimulation, they may give rise to impulses 

 that are conveyed to peripheral organs. This automatic stimulation 

 may be either continuous, that is persisting without interruption (tonic 

 automatism or tonus), or intermittent, pursuing a certain rhythm 

 (rhythmic automatism). 



The central organs are the trophic centers for the nerves passing out 

 from them. They may also act as centers for the nutrition of the tissues 

 innervated by them. Psychic activity is dependent on an intact con- 

 dition of the ganglionic central organs. 



The foregoing functions are related to different centers, none of which 

 is capable of representing several activities. 



THE SPINAL CORD. 



THE STRUCTURE OF THE SPINAL CORD. 



The spinal cord (Fig. 250) contains within its structure the gray matter, which 

 on section is H-shaped, and exhibits the anterior horns (co.d), the posterior 

 horns (co.p) and the central connecting segment constituted of the anterior and 

 the posterior gray commissures. In the middle of the last-named structure, from 

 the calamus scriptorius downward, passes the central canal, which is the remains 

 of the embryonal medullary tube and is lined by two or three layers of cylindrical 

 epithelial cells. 



The white matter surrounds the gray and it is divided into several columns. 

 In the median line, anteriorly, a deep fissure (s.a) extends into the cord, but 

 not quite to the gray matter, leaving at its bottom the white commissure (c.d) 

 intact. The anterior column (f.a) is situated between the anterior longitudinal 

 fissure and the groove for the exit of the anterior roots. The lateral portion of 

 the white matter between the anterior and posterior roots is known as the lateral 

 column (/./). Finally, the area between the line of exit of the posterior roots 

 and the posterior longitudinal fissure is designated the posterior column (f.p). 

 The posterior longitudinal fissure (s.p) penetrates more deeply than the anterior 

 into the cord, up to the gray matter. 



The white substance consists of medullated nerve-fibers provided with horny 

 sheaths, and arranged longitudinally in the columns. The incoming roots, as well 



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