Volume 3

A text book of physiology / by John Gray M'Kendrick ; including histology by Philipp Stöhr.

  • John Gray McKendrick
Date:
1888-1889
Catalogue details

Licence: Public Domain Mark

Credit: A text book of physiology / by John Gray M'Kendrick ; including histology by Philipp Stöhr. Source: Wellcome Collection.

    24/316 page 850
    of the spinal cord itself, it will be desirable to say a few words concerning the spinal nerves, that is to say the nerves which issue from the spinal cord. ^^e^X^^i^^^ We have already seen (§ 96) that each of the spinal nerves u.^ % Pc^T a.^ti arises by two roots, an anterior root attached to the ventral or ^ dc^o^n. u anterior surface, and a posterior root attached to the dorsal or ^^^^^ posterior surface of the cord. We have further seen that the latter bears a ganglion, a 'ganglion of the posterior root' or 'spinal ganglion,' and we have (§ 97) studied the structure of this ganglion. We stated at the same time that while the trunk of a spinal ^ nerve contained both efferent and afferent fibres, the efferent ^ ' 5f!X'r ^^^^^^ were gathered up into the anterior root and the afferent tt^ ^ r ^fibres into the posterior root; but we gave no proof of this state- ment. § 559. Before we proceed to do so, it will be as well to say a few words on the terms 'efferent' and 'afferent.' By efferent nerve {r'^'^^ fibres we mean nerve fibres which in the body usually carry impulses from the central nervous system to peripheral organs. Most efferent nerve fibres carry impulses to muscles, striated or plain, and the impulses passing along them give rise to movements; hence they are frequently spoken of as 'motor' fibres. But all ^ o.x« ^<fUy'\ efferent fibres do not end in or carry impulses to muscular fibres; <jL^xCi<y^ we have seen for instance that some efferent fibres are secretory. Moreover all the nerve fibres going to muscular fibres do not serve to produce movement; some of them, as in the case of certain vagus fibres going to the heart, are inhibitory and may serve to stop movement. ^ 0 nn f'-c 'afferent' nerve fibres we mean nerve fibres which in the H ioU'o body usually carry impulses from peripheral organs to the central nervous system. A very common effect of the arrival at the central nervous system of impulses passing along afferent fibres is that change in consciousness which we call a 'sensation'; hence afferent fibres or impulses are often called ' sensory' fibres or impulses. But as we have already in part seen, and as we shall shortly see in greater detail, the central nervous system may be affected by afferent impulses, and that in several ways, quite apart from the development of any such change of consciousness as may be fairly called a sensation. We shall see reason for thinking that afferent impulses reaching the spinal cord, and indeed other parts of the central nervous system, may modify reflex or automatic or other activity without necessarily giving rise to a sensation. Hence it is advisable to reserve the terms 'efferent' and 'afferent' as more general modes of expression than ' motor' or ' sensory.' We have seen in treating of muscle and nerve, that the changes produced in the muscle serve as our best guide for determining the changes taking place in a motor nerve; when a motor nerve is
    No text description is available for this image
    No text description is available for this image
    No text description is available for this image