THE CEREBEO-SPINAL AXIS. 489 



THE CEREBEO-SPINAL AXIS. 



Under this head it is intended to describe the Central Organs of the 

 K'ervous System, both as regards their general conformation and their 

 minute structure. The central ganglia of the sympathetic system have 

 been described along with the sympathetic nerves. 



The cerebrospinal axis is contained partly within the cavity of the 

 cranium, and partly within the vertebral canal ; it is divided by anato- 

 mists into the brain or encejilialon, and the enlarged upper mass placed 

 within the cranium, and the spinal cord contained within the vertebral 

 canal. It is symmetrical in its foiTa and structure throughout, con- 

 sisting 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 Avhite and grey nervous substance which cross from one 

 side to the other, 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 two 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 arachnoid ; and, 3rd, a highly vascular mem- 

 brane named the pia mater, which is next to, and closely invests the 

 surface of the brain and cord. 



A.— SPINAL CORD. 



EXTERNAL FORM. 



The spinal cord, or spinal marroiv (medulla spinalis), is that part of 

 the cerebro-spinal axis which is situated within the vertebral canal. It 

 extends from the margin of the foramen magnum of the occipital bone 

 to about the lower part of the body of the first lumbar vertebra. Above, 

 it is continued into the medulla oblongata ; below, it ends in a slender 

 filament, the filum terminale or central ligament of the spinal cord 

 (fig. 344 B'). 



Invested closely by a proper membrane (the pia mater), the cord is 

 enclosed within a sheath (theca) considerably longer and larger than 

 itself, formed by the dura mater, and separated from the walls of the 

 canal by numerous venous plexuses, and much loose areolar tissue 

 (fig. 345). Between the dura-matral sheath and the pia mater is 

 another membrane (the arachnoid), and between the latter and the pia 

 mater is a fluid called the cerebro-spinal fluid. Within this space the 

 cord is kept in position by proper ligaments, which fix it at different 

 points to its sheath, and by the roots of the spinal nerves, — an anterior 

 and a posterior root belonging to each, — which pass across the space 

 from the surface of the cord towards the intervertebral foramina. From 



