NEUROLOGY. OF THE GENERAL STRUCTURE THE nervous system is essentially constituted of nerve-cells and their processes. The processes are termed axons and dendrons or dendrites. Nerve-fibres are always axons of nerve-cells. The nervous substance proper is supported partly by connective tissue which brings its vascular supply, partly by a tissue peculiar to the nervous system termed neuroglia, consisting of special cells and fibres, which have an ectodermic origin in common with the nerve-cells. The nucleated bodies of the nerve-cells are contained in the grey matter of the nerve-centres and in the ganglia: their nerve-fibre processes form the white matter of the nerve-centres as well as the nerves, which are distributed to all parts of the body. Anatomically the nervous system is usually described as consisting of two parts-viz. the cerebrospinal system and the sympathetic system. The cerebrospinal system (fig. 1) comprises the spinal cord and the various divisions of the brain, together with the nerves which pass to and from the brain and spinal cord, and also the ganglia upon the roots of certain of these nerves (cerebrospinal ganglia). The spinal cord and brain constitute the central nervous system or cerebrospinal axis. The main divisions of the brain are from below upwards: (1) The hind-brain, comprising the spinal bulb or medulla oblongata (which is in direct continuation with the spinal cord), the pons, and the cerebellum; (2) the mid-brain, comprising the pedunculi cerebri and the corpora quadrigemina; (3) the fore-brain, including the thalami, the corpora striata, and the cerebral hemispheres. The sympathetic system (fig. 2) forms a chain of nerves and ganglia in front of the vertebral column which is connected with the thoracic and lumbar regions of the spinal cord (first thoracic to third lumbar segment) by fine nervous cords, the so-called white rami. From the sympathetic chain nerves pass to blood-vessels and viscera, and connected with these are other ganglia belonging to the same system. From the ganglia of the chain also other fine cords, the grey rami, pass to be distributed to all the spinal nerves, and along with these to the vessels, hair-muscles, and glands of the skin. The white rami are composed of very fine medullated fibres, which arise from cells in the lateral part of the grey matter of the spinal cord and are passing to the sympathetic ganglia, in one of which, either a ganglion of the chain or a more peripherally placed ganglion, they end. But, besides these white fibres which pass to the sympathetic, there are other nerves, similarly constituted of fine medullated fibres, VOL. III. B which emerge along with certain of the cerebral nerves-viz. the third, seventh, ninth, tenth, and eleventh-and yet others which emerge along with the ventral roots of the second and third sacral nerves. These do not proceed to join the sympathetic chain, but pass more directly to the visceral and vascular systems, to end in small ganglia near the periphery. But in their mode of origin and in the parts to which they are distributed, the fibres in question, which constitute the cranial and sacral splanchnic systems of Gaskell, closely resemble the thoracic and lumbar splanchnic nerve-fibres which pass to the sympathetic chain. They are therefore grouped by physiologists along with the sympathetic nerves and ganglia, and are distinguished by Langley under the term parasympathetic nerves; while to the whole system of sympathetic and parasympathetic nerve-fibres and ganglia the same author has given the name of autonomic nervous system. The nerves of the autonomic system differ from those of the cerebrospinal system in being formed of much finer fibres, and further in the fact that those fibres which arise in the ganglia proper to this system as a general rule lack the medullary sheath which gives to ordinary nerves their white appearance; so that these sympathetic nerve-fibres, which are proceeding to their distribution in the tissues, have a greyish colour (non-medullated fibres, fibres of Remak). The sympathetic (autonomic) ganglia differ from those of the cerebrospinal system in being situated not on nerve-roots but more peripherally; their cells also are different-each one usually FIG. 1.-VIEW OF THE CEREBROSPINAL AXIS. (After Bourgery.). The right half of the cranium and trunk of the body has been removed by a vertical section; the membranes of the right side of the brain and spinal cord have been cleared away, and the roots and first part of the fifth and twelfth cerebral nerves, and of all the spinal nerves of the right side, have been dissected out and laid separately on the wall of the skull and on the several vertebræ opposite to the place of their natural exit from the craniospinal cavity. F, Pa, T, O, frontal, parietal, temporal, and occipital lobes of cerebrum; C, cerebellum; P, pons; m o, placed behind medulla oblongata; m s, m s, point to the upper and lower extremities of the medulla spinalis; ce, on the last lumbar vertebral spine, marks the cauda equina; v, the three principal branches of the nervus trigeminus; C 1, the suboccipital or first cervical nerve; C vIII, the eighth or lowest cervical nerve; D 1, the first thoracic nerve; D XII, the last thoracic; LI, the first lumbar nerve; L v, the last lumbar; S 1, the first sacral nerve; S v, the fifth; Co 1, the coccygeal nerve; s, the left sacral plexus. FIG. 2. DIAGRAMMATIC VIEW OF THE SYMPATHETIC CORD OF THE RIGHT SIDE, SHOWING ITS CONNEXIONS WITH THE PRINCIPAL CEREBROSPINAL NERVES AND THE MAIN PRE-AORTIC PLEXUSES. Cerebrospinal nerves.-VI, a portion of the sixth cerebral nerve as it passes through the cavernous sinus, receiving two twigs from the carotid plexus of the sympathetic nerve; O, ciliary ganglion, connected by a twig with the cavernous plexus; M, connexion of the sphenopalatine ganglion by the large deep petrosal nerve with the carotid plexus; C, cervical plexus; Br, brachial plexus; D 6, sixth thoracic nerve; D 12, twelfth ; L 3, third lumbar nerve; S 1, first sacral nerve; S 3, third; S 5, fifth; Cr, femoral nerve; Cr', sciatic; pn, pneumogastric (vagus) nerve in the lower part of the neck; r, its recurrent branch, winding round the subclavian artery. Sympathetic cord.-c, superior cervical ganglion; c', middle; c", inferior; from each of these ganglia cardiac nerves (all deep on this side) are seen descending to the cardiac plexus; d 1, placed immediately below the first thoracic sympathetic ganglion; d 6, is opposite the sixth; 1, first lumbar ganglion; c g, the terminal or coccygeal ganglion. Pre-aortic and visceral plexuses.-pp, pharyngeal plexus; pl, posterior pulmonary plexus, spreading from the vagus on the back of the right bronchus; ca, on the aorta, the cardiac plexus, towards which, in addition to the cardiac nerves from the three cervical sympathetic ganglia, other branches are seen descending from the vagus and recurrent nerves; co, right or posterior, and co', left or anterior coronary plexus; o, oesophageal plexus in long meshes on the gullet; sp, great splanchnic nerve; +, small splanchnic; ++, smallest splanchnic; the first and second of these are shown joining so, the coeliac plexus; the third descending to re, the renal plexus; connecting branches between the coeliac, plexus and the vagus nerves are also represented; pn', above the place where the right vagus passes to the posterior surface of the stomach; pn", the left vagus, distributed on the anterior surface of the fundus of the organ; from the coeliac plexus large branches are seen surrounding the arteries of the coeliac axis, and descending to ms, the superior mesenteric plexus; opposite to this is an indication of the suprarenal plexus; below re (the renal plexus), the spermatic plexus is also indicated; ao, on the front of the aorta, marks the aortic plexus, formed by nerves descending from the coeliac and superior mesenteric plexuses and from the lumbar ganglia; mi, the inferior mesenteric plexus, surrounding the corresponding artery; hy, hypogastric plexus, placed between the common iliac arteries, connected above with the aortic plexus, receiving nerves from the lower lumbar ganglia, and dividing below into the right and left pelvic or inferior hypogastric plexuses; pl, right pelvic plexus; from this the nerves descending are joined by those from the plexus on the superior hæmorrhoidal vessels, mi', by sympathetic nerves from the sacral ganglia, and by numerous visceral nerves from the third and fourth sacral spinal nerves, and there are thus formed the rectal, vesical, and other plexuses, which ramify upon the viscera from behind forwards, and from below upwards, as towards ir, and v, the rectum and bladder. |