576 FUNCTIONS OF THE CEREBRO -SPIN AL NERVOUS SYSTEM. 



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

 from their apparent ones. 



466. Most important information as to the functions of particular nerves 

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

 fallacious results, unless they are prosecuted with a full knowledge of all the 

 precautions 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 in- 

 osculation from another nerve ( 463). 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 ex- 

 citation of muscular movements by irritation of its trunk, whilst still in con- 

 nection 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 stimu- 

 lation acting through the ganglionic centre upon some other nerve. The 

 real nature of such movements can only be determined by dividing the trunk, 

 and then 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 icith 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 Glosso- 

 pharyngeal has been satisfactorily determined, by experiments of this kind, 

 performed by Dr. J. Reid ( 89), to be chiefly, if not entirely, an allerent 

 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 division of either root, 

 motions are often excited ; but it 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 centre, 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 exhibits signs 

 of pain, it may be concluded that the nerve is capable of receiving and 

 transmitting sensory impressions from its peripheral extremity. But it not 

 unfrequently happens that this capability is derived by inosculation with 

 another nerve ; as is the case with the Facial, which is sensory after it has 

 passed through the parotid gland, having received there a twig from the 

 Fifth pair. 



467. The fallacies to which all experiments upon the nerves are subject, 

 arising from the partial loss of their power of receiving and conveying im- 

 proHons, and of exciting the muscles to action, after death, are too obvious 

 to require more particular mention here ; yet they are frequently overlooked. 

 Of a similar description are those arising from severe disturbance of the 

 system, in consequence of operations; which also have not been enough re- 

 gard* (I by experimenters. As a general rule, negative results are of less value 

 than positive, but very careful discrimination is often required to determine 

 what nre negative, and what positive results. Kach particular case has its own 

 sources of fallacy, which require to be logically scrutinized ; and the only sal is- 

 factory proof is derived from the concurrence of every kind of evidence which 



