MODE OF DETERMINING THE FUNCTIONS OF NERVES. 97 



mon sensibility. The case is different, however, in regard to the sense of 

 taste, which originates in impressions not far removed from those of ordinary 

 touch; and it is probable that the same nerves minister to both. Anatomical 

 evidence of this kind is valuable also, not only in reference to the functions of 

 a principal trunk, but even as to those of its several branches, which, in some 

 instances, differ considerably. Thus, some of the branches of the par vagum 

 are especially motor, and others almost exclusively afferent; and anatomical 

 examination, carefully prosecuted, not only assigns the reasons for these func- 

 tions, when ascertained, but is in itself nearly sufficient to determine them. 

 Thus the superior laryngeal nerve is distributed almost entirely upon the 

 mucous surface of the larynx, the only muscle it supplies being the crico- 

 thyroid ; whilst the inferior laryngeal or recurrent is almost exclusively dis- 

 tributed to the muscles. From this we should infer that the former is an 

 afferent, and the latter a motor nerve ; and experimental inquiries (hereafter 

 to be detailed) fully confirm this view. In like manner it may be shown, 

 that the glosso-pharyngeal is chiefly an afferent nerve, since it is distributed 

 to the surface of the tongue and *pharynx, and scarcely at all to the muscles 

 of those parts ; whilst the pharyngeal branches of the par vagum are chiefly, 

 if not entirely, motor. Lower down, however, the branches of the glosso- 

 pharyngeal cease, and the oesophageal branches of the par vagum are dis- 

 tributed both to the mucous surface and to the muscles ; from which it may 

 be inferred that they are both afferent and motor a deduction which experi- 

 ment confirms. 



121. We perceive, therefore, that much knowledge of the function of a 

 nerve may be obtained from the attentive study of its ultimate distribution : 

 but it is necessary that this should be very carefully ascertained, before it is 

 made to serve as the foundation for physiological inferences. As an example 

 of former errors in this respect, may be mentioned the description of the 

 pprtio dura of the seventh, at first given by Sir C. Bell : he stated it to be 

 distributed to the skin as well as to the muscles of the face, and evidently 

 regarded it as in part an afferent nerve, subservient to respiratory impressions 

 as well as to motions. In the same manner, from inaccurate observation of 

 the ultimate distribution of the superior laryngeal nerve, it was long regarded 

 as that which stimulated to action the constrictors of the glottis. But the 

 knowledge obtained by such anatomical examinations alone is of a very gene- 

 ral kind; and requires to be made particular, to be corrected and modified, 

 by other sources of information. One of these relates to the connection of 

 the trunks with the central organs. The evidence derived from this source, 

 however, is seldom of a very definite character ; and, in fact, the functions of 

 particular divisions of the nervous centres have been hitherto rather judged of 

 by those of the nerves with which they are connected, than affording aid in the 

 determination of the latter. Still, this kind of examination is not without its 

 use, when there is reason to believe that a particular tract of fibrous structure 

 has a certain function, and when the office of a nerve whose roots terminate 

 in it is doubtful. Here, again, however, very minute and accurate examina- 

 tion is necessary, before any sound physiological inferences can be drawn from 

 facts of this description ; and many instances might be adduced to show, that 

 the real connections of nerves and nervous centres are often very different 

 from their apparent ones. 



122. Experimental inquiries into the functions of particular nerves are also 

 liable to give fallacious results, unless they are prosecuted with a full know- 

 ledge of all the precautions necessary to insure success. Some of these will 

 be here explained. Suppose that, upon irritating the trunk of a nerve, whilst 

 still in connection with its centre, muscular movements are excited ; it must 

 not be hence concluded that the nerve is an efferent one, for it may have no 



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