GENERAL ARRANGEMENT OF NERVOUS SYSTEM. 433 



Fig. 147. 



complete symmetry of arrangement. The internal abdominal organs 

 are in great measure unsymmetrical, and the corresponding nerves and 

 nervous centres of the ganglionic S3'stem present the same want of 

 regularity in their locality and distribution. But while the ganglionic 

 system presides over the internal organs and 

 functions of nutrition, the cerebro-spinal sys- 

 tem, on the other hand, is connected with the 

 apparatus of animal life, namely, the organs 

 of sensation and movement by which the 

 living body is brought into relation with the 

 exterior. As these organs, in man and the 

 vertebrate animals, are symmetrically ar- 

 ranged, tlie cerebro-spinal nervous system 

 presents the same character. Both the brain 

 and the spinal cord are composed of two, 

 right and left, lateral halves ; each one of 

 which furnishes the nerves of sensation and 

 motion to the corresponding sides of the 

 body. 



Another striking peculiarity of this part 

 of the nervous system is the mutual decus- 

 sation of the nerve fibres belonging to its 

 two sides. Both the brain and spinal cord 

 have their right and left halves connected 

 by fibres which pass across the median line 

 from one to the other ; the different bundles 

 being often interwoven with eacli other, at 

 the point of transit, in a somewhat compli- 

 cated manner. This peculiarity extends to 

 the origins of the nerves, which decussate 

 with each other internally ; so that the nerve 

 fibres emerging from the right side of the 

 cerebro-spinal mass have their origin from 

 the gray substance of the left lateral half, 

 and those emerging from the left side origi- 

 nate from the gray substance of the right 

 lateral half. The only uncertainty in this 

 respect is whether the decussation be com- 

 plete or partial; that is, whether all the 

 fibres of a given nerve root be connected 

 with the opposite side of the central mass, or 

 whether a part of them originate from the 

 same and a part from the opposite side. The decussating fibres, in a 

 large number of instances, are anatomically demonstrated. In some 

 remaining exceptions their course is more or less a matter of doubt. 



The Spinal Cord is a nearly cylindrical nervous mass, inclosed in the 

 cavity of the spinal canal, commencing by a slightly enlarged extremity 



THE BRAIN AND SPINAL 

 CORD, in profile. 



