Nervous System. 



THE Nervous System is composed: 1, of a series of connected central 

 organs, called, collectively, the cerebro- spinal centre, or axis; 2, of the ganglia; 

 and, 3, of the nerves. 



The Cerebro-spinal Centre consists of two parts, the spinal cord and the 

 encephal-on: the latter may be subdivided into the cerebrum, the cerebellum, 

 the pens Varolii, and the medulla oblongata. 



THE SPINAL CORD AND ITS MEMBRANES. 



Dissection. To dissect the cord and its membranes, it will be necessary to lay open the 

 whole length of the spinal canal. For this purpose, the muscles must be separated from the 

 vertebral grooves, so as to expose the spinous processes and laminae of the vertebrae ; and the 

 latter must be sawn through on each side, close to the roots of the transverse processes, from 

 the third or fourth cervical vertebra, above, to the sacrum below. The vertebral arches having 

 been displaced, by means of a chisel, and the- separate fragments removed, the dura mater will 

 be exposed, covered by a plexus of veins and a quantity of loose areolar tissue, often infiltrated 

 with a serous fluid. The arches of the upper vertebrae are best divided by means of a strong 

 pair of cutting bone-forceps. 



MEMBRANES OF THE CORD. 



The Membranes which envelop the spinal cord are three in number. The 

 most external is the dura mater, a strong fibrous membrane, which forms a 

 loose sheath around the cord. The most internal is the pia mater, a cellulo- 

 vascular membrane, which closely invests the entire surface of the cord. 

 Between the two is the arachnoid membrane, an intermediate serous sac, which 

 envelops the cord, and is then reflected on the inner surface of the dura mater. 



The Dura Mater'of the cord, continuous with that which invests the brain, 

 is a loose sheath which surrounds the cord, and is separated from the bonv 

 walls of the spinal canal by a quantity of loose areolar adipose tissue, and a 

 plexus of veins. It is attached to the circumference of the foramen magnum, 

 and to the posterior common ligament, throughout the whole length of the 

 spinal canal, by fibrous tissue, and extends, below, as far as the top of the 

 sacrum; but, beyond this point, it is impervious, being continued, in the form 

 of a slender cord, to the back of the coccyx, where it blends with the perios- 

 teum. This sheath is much larger than is necessary for its contents, and its 

 size is greater in the cervical and lumbar regions than in the dorsal. Its inner 

 surface is smooth, covered by a layer of polygonal cells; and on each side may 

 be seen the double openings which transmit the two roots of the corresponding 

 spinal nerve, the fibrous layer of the dura mater being continued in the form 

 of a tubular prolongation on them as they issue from these apertures. These 

 prolongations of the dura mater are short in the upper part of the spine, but 

 become gradually longer below, forming a number of tubes of fibrous mem- 

 brane, which inclose the sacral nerves, and are contained in the spinal canal. 



The chief peculiarities of the dura mater of the cord, as compared with that 

 investing the brain, are the following: 



The dura mater of the cord is not adherent to the bones of the spinal canal, 

 which have an independent periosteum. 



It does not send partitions into the fissures of the cord, as in the brain. 



Its fibrous lamina do not separate, to form venous sinuses, as in the brain. 



Structure. The dura mater consists of white fibrous tissue, arranged in 

 572 



