CHAPTER III. 

 GENERAL ARRANGEMENT OF THE NERVOUS SYSTEM. 



THE nervous system, in man and the higher animals, includes two 

 secondary systems, or groups of nervous centres, with their com- 

 missural fibres and peripheral nerves. They are : first, the cerebro- 

 spinal system, presiding over the functions of animal life ; and, sec- 

 ondly, the sympathetic system, connected with the internal acts of 

 nutrition. 



Cerebro-Spinal System. Of the two groups above mentioned, the 

 cerebro-spinal system largely preponderates by its mass, the well- 

 marked distinction of its parts, and the striking character of its phe- 

 nomena. Its centres are also, in great measure, the sources of func- 

 tional activity for the sympathetic system, and thus control, directly 

 or indirectly, the nervous relations of the whole body. As its name 

 indicates, the nervous centres belonging to it are the brain and the 

 spinal cord; the nerves which originate from them being distributed 

 to the muscles and tegumentary surfaces of the head, trunk, and limbs, 

 to the organs of special sense, and to the commencement and termina- 

 tion of the internal passages of the body. 



In its general form the cerebro-spinal system is distinguished by a 

 nearly complete bilateral symmetry. Like the organs of animal life 

 over which it presides, it consists of a double series of corresponding 

 structures, united with each other upon the median line. This union 

 is effected by transverse commissures; that is, by fibrous tracts pass- 

 ing from side to side between similar parts and enabling them to act 

 in harmony with each other. The right and left halves of the brain 

 and spinal cord, thus connected, furnish the nerves of sensation and 

 motion to the two sides of the body. 



Another peculiarity of this portion of the nervous system is the 

 decussation of its fibres. By this term is meant an oblique passage 

 of fibres across the median line, forming a connection between dis- 

 similar parts on the two sides; and as this oblique crossing takes 

 place simultaneously from right to left and from left to right, the two 

 tracts of decussating fibres are interwoven with each other at the 

 median line. This is most distinctly shown in the decussation of the 

 optic nerves at the base of the brain and in that of the anterior pyra- 

 mids at the medulla oblongata ; but other instances occur at various 

 points in the interior, and it may be said, in general, that the nerves 

 emerging from the right side of the cerebro-spinal mass have their 



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