NEUROLOGY. CEREBRO-SPINAL AXIS. 501 



SECTION V. NEUROLOGY. 



UNDER the name of Neurology, it is intended to include the descriptive 

 anatomy of the various organs forming parts of the nervous system. 



The nervous system consists of two sets of parts, one of which is eentral, 

 the other peripheral. To the first set belong the brain and spinal cord, 

 forming together the cerebro-spinal axis, and the ganglia : to the second 

 set belong all the nerves distributed throughout the body ; and along with 

 these may be included the organs of the senses, or those organs which 

 contain the terminations of the several nerves of special sensation, in con- 

 nection with certain apparatus or modifications of structure related to the 

 reception of impressions by each of these nerves. 



Among the peripheral nerves it is necessary also to distinguish the cere- 

 bro-spinal and the sympathetic or ganglionic, which, though intimately 

 connected with each other at some places, are yet so different in their 

 structure and mode of distribution as to require separate description. 



The description of these several parts of the nervous system will be 

 brought under the following four subsections, viz. 1. The cerebro-spinal 

 axis ; 2. The cerebro-spinal nerves and the ganglia connected with them ; 

 3. The sympathetic nerves and their ganglia ; 4. The organs of the 

 senses. 



I. THE CEREBRO-SPINAL AXIS. 



The cerebro-spinal axis is contained partly within the cavity of the cra- 

 nium, and partly within the vertebral canal ; it is symmetrical in its form 

 and structure throughout, consisting of a right and a left half, separated 

 to a certain extent by longitudinal fissures, and presenting in their plane of 

 union various portions of white and grey nervous substance which cross 

 from one side to another, and form the commissures of the brain and spinal 

 cord. 



Enclosed within the skull and the vertebral canal, the cerebro-spinal axis 

 is protected by the bony walls of those tw6 cavities ; it is also surrounded 

 by three membranes, which afford it additional protection and support, and 

 are subservient to its nutrition. These envelopes, which will be described 

 hereafter, are, 1st, a dense fibrous membrane named the dura mater, which 

 is placed most superficially ; 2nd, a serous membrane called the arach- 

 noid ; and, 3rd, deepest of all, a highly vascular membrane named the pia 

 mater. 



The cerebro-spinal axis is divided by anatomists into the encephalon or 

 enlarged upper mass placed within the cranium, and the spinal cord con- 

 tained within the vertebral canal. 



These two parts have a relation, one to the other, very similar to that 

 which subsists between the cranium and vertebral column : thus, they are 

 continuous structures ; at the time of their first formation in the foetus they 

 are nearly similar ; the earliest developed distinction consists in the enlarge- 

 ment of the encephalon ; and, moreover, the spinal cord, like the vertebral 

 column, continues to present a structure nearly uniform throughout its 

 extent, while the encephalon becomes gradually more and more complicated, 



