GENERAL SUMMARY. 655 



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. 



691. 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 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, Physiolo- 

 gists have rather been accustomed to judge of the functions of particular divi- 

 sions 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 the real connections of nerves and nervous cen- 

 tres are often very different from their apparent ones. 



692. Most important information as to the functions of particular nerves may 

 be drawn from experimental inquiries; but these also are liable to give falla- 

 cious results, unless they are prosecuted with a full knowledge of all the precau- 

 tions necessary to insure success. Some of these will be here explained. In 

 the first place, the endowments of the trunk and of the roots of a nerve may 

 differ; owing to the admixture, in the former, of fibres derived by inosculation 

 from another nerve ( 688). Hence, in order to attain satisfactory results, a 

 comparative set of experiments should always be made upon each. A nerve- 

 trunk may be too hastily considered as motor, on account of the excitation of 

 muscular movements by irritation of its trunk, whilst still in connection with its 

 centre; for such movements may be called forth, not only by the direct influence 

 of the nerve upon the muscles, but also by reflex stimulation, acting through the 

 ganglionic centre upon some other nerve. The real nature of such movements 

 can only be determined by dividing the trunk, and irritating each of the cut 

 extremities. If, upon irritating the end separated from the centre, muscular 

 contractions are produced, it may be safely inferred that the nerve is, in part at 

 least, of an efferent character. Should no such result follow, this would be 

 improbable. If, on the other hand, muscular movement should be produced by 

 irritating the extremity in connection with the centre, it will then be evident, 

 that it is occasioned by an impression conveyed towards the centre by this 

 trunk, and propagated to the muscles by some other; in other words, to use 

 the language of Dr. M. Hall, this nerve is an " excitor" of motion, not a direct 

 motor nerve. The Grlosso-pharyngeal has been satisfactorily determined, by 

 experiments of this kind, performed by Dr. J. Reid ( 427), to be chiefly, if not 

 entirely, an afferent nerve. It has been from the want of a proper mode of 

 experimenting that the functions of the posterior roots of the Spinal nerves 

 have been regarded as in any degree motor. If they be irritated, without divi- 

 sion of either root, motions are often excited ; but if they be divided, and their 

 separated trunks be then irritated, no motions ensue; nor are any movements 

 produced by irritation of the roots in connection with the spinal cord, if the ' 

 anterior roots have been divided. Hence it appears that these fibres do not 

 possess any direct motor powers, but that they convey impressions to the cen- 

 tre, which are reflected to the muscles through the anterior roots. The same 

 difficulties do not attend the determination of the sensory endowments of nerves. 

 If, when the trunk of a nerve is pricked or pinched, the animal exhibit signs 



