Human physiology / by John Thornton.

  • Thornton, John
Date:
1899
Catalogue details

Licence: Public Domain Mark

Credit: Human physiology / by John Thornton. Source: Wellcome Collection.

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    days. Soon after section a gradual degeneration begins in the peripheral fibres thus cut oft' from the cells to which they belong, and which govern their nutrition. The white medullary sheath breaks up into small granular oily-looking masses and at length disappears. The axis cylinder also breaks up, and this also is absorbed, so that all that is left of .the severed nerve is the primitive sheath with protoplasntic contents. This degenera- tion takes place along the whole of the severed nerve simultaneously, and the process las<s about ten or twelve days. (Degeneration of a nerve after section is often called, from its discoverer, Wallerian degeneration.) The cut end connected with the centre does not degenerate, but in the course of tw^o or three months regeneration of the nerve has begun by the growth from the central stump of new axis cylinders, a repetition of the process of the development of the nerve. These pass along the old track and form the axis cylinders of new nerve-fibres, which in time become surrounded with a medullary sheath, and reaching the periphery may restore the lost excitability of the part. From what has just been said it is evident that section of a spinal nerve results in degeneration of all the nerve-fibres leading to the periphery because all these have their trophic centres near the cord—the afferent fibres being connected with cells in the ganglion of the posterior root, the efferent fibres with cells in the anterior horn of grey matter in the cord (par. 51). 48. Functions of Nerves.—We have already said that nerves are divided into two great classes according as they conduct impulses from a nerve centre (efferent) or to a nerve centre (afferent). From what has been said it will be under- stood that the function of a nerve is ascertained by determining its mode of origin, both superficial and deep, its mode of distribution to muscle or gland or sense organ, and its physiological behaviour. If it spring from a tract of matter where other afferent or efferent nerves arise, its general function may be surmised ; if it be distributed to a muscle or gland it is evidently motor or efferent; if it terminate in a sense organ in such a way as to be adapted to receive particular impressions, it must be sensory or afferent. But experiment and observation of the nerve under varying conditions will be the best guide. It may be stimulated in its course and the effects observed, or it may be divided and the peripheral and central ends stimulated separately. Stimulation of the end going to the periphery would show whether this led to contraction of muscle, secretion of gland, arrest or increase of action. Stimulation of the end connected with the nerve centre might show whether a sensation pf any kind was set up. Finally, examination of the cut end under the microscope would show in what direction degenera-
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