500 NERVOUS SYSTEM. 



centre, is called the apparent origin of a nerve ; the term, real or deep origin, being 

 given to that part of the centre from which a nerve actually springs. 



The Sympathetic nerve consists of tubular and gelatinous fibres, intermixed 

 with a varying proportion of filamentous areolar tissue, and inclosed in a sheath 

 formed of fibro-areolar tissue. The tubular fibres are, for the most part, smaller 

 than those composing the cerebro-spinal nerves; their double contour is less 

 distinct, and, according to Eemak, they present nuclei similar to those found in 

 the gelatinous nerve fibres. Those branches of the sympathetic which present a 

 well-marked gray color are composed more especially of gelatinous nerve fibres, 

 intermixed with few tubular fibres ; whilst those of a white color contain more 

 of the tubular fibres, and few gelatinous. Occasionally the gray and white cords 

 run together in a single nerve, without any intermixture, as in the branches of 

 communication between the sympathetic ganglia and the spinal nerves, or in the 

 communicating cords between the ganglia. 



The nerve fibres both of the cerebro-spinal and sympathetic system convey 

 impressions of a twofold kind. The sensitive nerves, called also centripetal or 

 afferent nerves, transmit impressions made upon their peripheral extremities to the 

 nervous centres, and in this way the mind, through the medium of the brain, 

 becomes conscious of external objects. The motor nerves, called also centrifugal 

 or efferent nerves, transmit impressions from the nervous centres to the parts to 

 which the nerves are distributed, these impressions either exciting muscular con- 

 tractions, or influencing the processes of nutrition, growth, and secretion. 



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

 phalon : the latter may be subdivided into the cerebrum, the cerebellum, the pons 

 Varolii, and the medulla oblongata. 



THE SPINAL COED 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 Iamina3 of the vertebra? ; and the hitter 

 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 dis- 

 placed, 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 vertebra are best divided by means of a strong 

 pair of 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 envelopes 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 it, being separated from the bony 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 periosteum. This sheath is much larger 

 t'nan 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, and covered by a layer of 

 polygonal cells ; and on each side may be seen the double openings which trans- 

 mit the two roots of the corresponding spinal nerve, the fibrous layer of the dura 



