DETERMINATION OF THE FUNCTIONS OF NERVES. 575 



being partly sensory. Further, where a nerve is entirely distributed upon a 

 surface adapted to receive impressions or' 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 posse>s com- 

 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. Anatom- 

 ical evidence of this kind is valuable also, not only in reference to the func- 

 tions of a principal trunk, but even as to those of its several brandies, which, 

 in some instances, differ considerably. Thus, some of the branches of the 

 Pneumogastric are especially motor, and others almost exclusively afferent; 

 and anatomical examination, carefully prosecuted, not only assigns the rea- 

 sons for these functions, when ascertained, but is in itself nearly .sufficient to 

 determine them. For the superior laryngeal branch 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 alm.ist 

 exclusively distributed to the muscles. From this we might infer, that the 

 former is an afferent, and the latter a motor nerve ; and experimental in- 

 quiries (as we have seen, 301) 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 Pueumo- 

 gastric are chiefly, if not entirely, motor ( 89). Lower down, however, 

 the branches of the Glosso-pharyngeal cease, and the oesophageal branches 

 of the Pneumogastric 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 ( 90). We perceive, therefore, that 

 much knowledge of the function of a nerve may be obtained from the atten- 

 tive 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 (or 

 Facial ) at first given by Sir C. Bell ; for he incorrectly stated it to ba dis- 

 tributed to the skin as well as to the muscles of the face, and erroneously 

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

 sions as well as to motions. In the same manner, from inaccurate observa- 

 tion of the ultimate distribution of the Superior Laryugeal nerve, it was 

 long regarded as that which stimulated to action the constrictors of the 

 glottis. 



465. But the knowledge obtained by such anatomical examinations alone 

 is of a very general kind ; and requires to be made particular, to be cor- 

 rected and modified, by other sources of information. One of those 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, Physiologists have rather been accustomed to judge of the functions of 

 particular divisions of the nervous centres by those of the nerves with which 

 they are connected, than to draw aid from the former 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 physiological inferences can be drawn from 

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



