654 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



but also in that which is continuous with the other branch, as will be made 

 apparent by the contraction in the muscles supplied by the latter. That this 

 experiment may be free from the possible fallacy resulting from the excitement 

 of reflex action, the trunk of the sciatic nerve should be divided high up, or the 

 spinal cord be destroyed. 



690. Various methods of determining the functions of particular nerves 

 present themselves to the Physiological inquirer. One source of evidence is 

 drawn from their peripheral distribution. For example, if a nervous trunk is 

 found to lose itself entirely in the substance of Muscles, it may be inferred to 

 be chiefly, if not entirely, motor or efferent. In this manner, Willis long ago 

 determined that the Third, Fourth, Sixth, Portio dura of the Seventh and Ninth 

 cranial nerves, are almost entirely subservient to muscular movement ; and the 

 same had been observed of the fibres proceeding from the small root of the 

 Fifth pair, before Sir C. Bell experimentally determined the double function 

 of that division of the nerve into which alone it enters. Again, where a nerve 

 passes through the muscles, with little or no ramification among them, and 

 proceeds to a Cutaneous or Mucous surface, on which its branches are minutely 

 distributed, there is equal reason to believe that it is of a sensory, or rather of 

 an afferent j character. In this manner, Willis came to the conclusion that the 

 Fifth pair of cranial nerves differs from those previously mentioned, in being 

 partly sensory. Further, where a nerve is entirely distributed 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 con- 

 siderably. 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 reasons for these functions when 

 ascertained, but is in itself nearly sufficient to determine them. For the supe- 

 rior 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 almost exclusively distributed to the muscles. 

 From this we might infer that the former is an afferent, and the latter a motor 

 nerve; and experimental inquiries (as we have seen, 558) 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 Pneumogastric are chiefly, if not entirely, motor ( 427). Lower down, how- 

 ever, 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 ( 428). 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, 

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



