THE NERVOUS SYSTEM. 



THE Xervous System is composed 1. Of a series of large centres of nerve-matter, 

 called, collectively, the cerebro-spinal centre or axis. 2. Of smaller centres, 

 termed ganglia. 3. Of nerves* connected either with the cerebro-spinal axis or 

 the ganglia. And 4. Of certain modifications of the peripheral terminations of the 

 nerves, forming the organs of the external senses. 



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

 encephalon : the latter may be subdivided into the cerebrum, the cerebellum, the 

 pons 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 erooves. 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 tbe 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 

 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 dur" m.-it, /. 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 



iinoid membrane, a non-vascular membrane which envelops the cord and is 

 connected to the pia mater by slender filaments of connective tissue. 



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 bony walls of 

 the spinal canal by a quantity of loose areolar tissue and a plexus of veins. It 

 is attached to the circumference of the foramen magnum and to the posterior 

 common ligament, especially at the lower end of the spinal canal, by fibrous 

 slips, and extends below as far as the third piece 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 periosteum. 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. 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 pass through these apertures. On opening the 

 lower part of the dura mater i. e. below the termination of the cord proper 

 the root* of the lumbar and sacral nerves are seen. These roots, taken together, 

 form what is known as the cauda equina. In the midst of the cauda equina is a 

 delicate process of gray matter within a tube of pia mater. This is the filnm 

 terminal?. This comes off from the conus terminalis (Fig. 402) or cone-like end- 

 ing of the cord, and blends, below, with the slender cord-like prolongation of the 

 dura mater just mentioned. (See page 695.) 



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