DETERMINATION OF FUNCTIONS OF NERVES. 243 



elusion, that the fifth pair of cranial nerves differs from those previously 

 mentioned, in being partly sensory. Further, where a nerve is entirely dis- 

 tributed upon a surface adapted to receive impressions of a special kind, as 

 the Schneiderian membrane, the retina, or the membrane lining the internal 

 ear, it may be inferred that it is not capable of transmitting any other kind 

 of impressions; for experiment has shown, that the special sensory nerves do 

 not possess common 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 functions, when ascertained, but is in itself nearly sufficient to determine 

 them. Thus the superior laryngeal branch is distributed almost entirely upon 

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

 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-pharyn- 

 geal cease, and the ossophageal branches of the par vagum are distributed 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 experiment confirms. 



301. 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 

 Portio 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 re- 

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

 the knowledge obtained by such anatomical examinations alone is of a very 

 general kind; and requires to be made particular, to be corrected and modi- 

 fied 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 rather been 

 hitherto judged of, by those of the nerves with which they are connected, 

 than afforded 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 examination is necessary, before any sound physio- 

 logical 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. 



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



