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Functions of the Spinal Cord and Spinal Nerves

root, anterior, fibres, nerve, posterior, cells and impulses

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From what has been already said, we under stand that the spinal cord consists of (1) a series of ganglionic masses of nerve-cells, each regn biting, by reflex action through their related afferent or sensory and efferent or motor nerve fibres, the actions going on in a part of the body, and (2) of strands of nerve-fibres con veying sensory impulses up to the brain, and motor impulses down from the brain. A few more details of these functions will now be given. First as to the spinal nerves.

Functions of Spinal Nerves.—Each spinal nerve arises by two roots (see diagram, Fig. 96). If one anterior root of one side be cut, there is loss of the power of movement in the parts supplied by the nerve, and changes in the blood- vessels and glands of the part ; that is to say, by the anterior root impulses issue from the cord regulating the contraction of muscles, and controlling the blood-vessels and glands. In other words the anterior root is motor. If a posterior root be divided there is loss of sensa tion in the part supplied by the nerve, and no disturbance of movement. Stimulation of the upper end of the divided anterior root produces no result, stimulation of the lower end causes muscular movements. Stimulation of the upper end of the divided posterior root elicits indica tions of pain, while stimulation of the lower end produces no obvious effect. The spinal nerve is, therefore, a mixed nerve, containing both afferent or sensory and efferent or motor fibres. Along the afferent fibres impulses pass to the cord, entering its posterior portion, so reaching the ganglion cells, which, thus excited, discharge impulses by the fibres of the anterior root, at the front of the cord, down the nerve to muscles, vessels, and glands. Any injury of the anterior root which separates it from the cord is followed by degeneration—wasting and decay—of the motor fibres of the mixed spinal nerve below the seat of injury. So the course of these fibres can be traced to their ultimate terminations and distinguished from the course of the sensory fibres of the mixed nerve, which remain unaffected. This is because the fibres have been separated from their nerve-cells in the cord, which, therefore, besides their other functions, preside over the nutrition of the nerve-fibres of the anterior root. These nerve

cells are those of the anterior horn of the same side. Disease destroying these cells is followed by degeneration of the fibres of the anterior root issuing from them, and paralysis of the muscles supplied by them. On the posterior root is a ganglion (q, Fig. 96); division of the root outside of the ganglion is followed by degeneration of the nerve - fibres beyond and throughout the whole course of the mixed spinal nerve, fibres of the mixed nerve derived from the anterior root remaining unaffected. Division of the posterior root between the ganglion and the cord is followed by degenera tion of the part left connected with the cord, and the degeneration may in time be traced right up the whole length of the cord in its posterior portion, while the part of the root left connected with the ganglion, and all beyond the ganglion, remain unaffected. The nutrition of the fibres of the posterior root is, therefore, presided over by the cells of the ganglion of the posterior root.

Functions of the Spinal or afferent impulses enter the cord by the posterior root. These impulses find their way to the cells of the anterior cornu of the same side, though no direct anatomical continuity has been traced between the fibres of the posterior root and the cells. The cells thus stimulated dis charge impulses along the efferent fibres of the anterior root, which reaching muscles, &c., lead to movement. This is the reflex me chanism of the cord. The cells of the anterior horn give off numerous pro cesses which branch, and are lost in the fine meshwork of the cord, but there is always one process of the cell which issues from the horn, and be comes the central or axis cylinder of a nerve-fibre of the anterior root, so that a motor nerve-fibre which reaches a muscle, and conveys to it a stimulus leading to contraction, is a direct con tinuation of a process of a nerve-cell in the anterior horn of the same side of the spinal cord.

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