244 FUNCTIONS OF THE NERVOUS SYSTEM. 



liable to give fallacious results, unless they are prosecuted with a full know- 

 ledge of all the precautions necessary to insure success. Some of these will 

 be here explained. Suppose that, upon irritating the trunk of a nerve, whilst 

 still in connection with its centre, muscular movements are excited ; it must 

 not be hence concluded that the nerve is an efferent one, for it may have no 

 directly motor powers. The next step would be to divide the trunk, and to 

 irritate 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 doubtful. If, on the other hand, muscular move- 

 ment should be produced by irritating the extremity in connexion 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 glosso-pharyngeal nerve 

 has been satisfactorily determined to be chiefly, if not entirely, an efferent 

 nerve, by experiments of this kind, performed by Dr. J. Reid. 



303. 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 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 connexion with the spinal cord, if the anterior roots 

 have been divided. Hence it appears that the motor powers of these fibres 

 are not direct, but that they convey an impression to the centre, which is 

 reflected to the muscles through the anterior roots. Another source of fallacy 

 is to be guarded against, arising from the communication to a nerve, in its 

 course, of properties it did not possess at its root, by inosculation with an- 

 other nerve. Of this many instances will hereafter present themselves. 



304. The same difficulties do not attend the determination of the sensory 

 properties of nerves. If, when the trunk of a nerve be pricked or pinched, 

 the animal exhibits signs of pain, it may be concluded that the nerve is sen- 

 sible to ordinary impressions at its peripheral extremity. But not unfre- 

 quently this sensibility is derived by inosculation with another nerve; as is 

 the case with the portio dura, which is sensory after it has passed through 

 the parotid gland, having received there a twig from the fifth pair. A similar 

 inosculation explains the apparent sensibility of the anterior roots of the 

 spinal nerves. If these be irritated, the animal usually gives signs of uneasi- 

 ness; but if they be divided, and the cut ends nearest the centre be irritated, 

 none such are exhibited ; whilst they are still shown, when the farther ends 

 are irritated, but not if the posterior roots are divided. This seems to indi- 

 cate that, from the point of junction of the two roots, sensory fibres derived 

 from the posterior root pass backwards (or towards the centre) in the anterior; 

 and thus its apparent sensory endowments are entirely dependent upon its 

 connexion with the posterior column of the spinal cord, through the posterior 

 roots. 



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

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

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

 to require 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 regarded by 

 experimenters. 



306. All our positive knowledge of the functions of the Nervous System 

 in general, save that which results from our own consciousness of what passes 



