98 FUNCTIONS OF THE NERVOUS SYSTEM. 



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 fol- 

 low, this would be doubtful. 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 exciter 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. 



123. 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 connection 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 another nerve. Of this 

 many instances will hereafter present themselves. 



124. 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 sensi- 

 ble to ordinary impressions at its peripheral extremity, feut not unfrequently 

 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 inoscula- 

 tion explains the apparent sensibility of the anterior roots of the spinal nerves. 

 If these be irritated, the animal usually gives signs of uneasiness ; but if they 

 be divided, and the cut ends nearest the centre be irritated, none such are ex- 

 hibited; whilst they are still shown, when the farther ends are irritated, but 

 not if the posterior roots are divided. This seems to indicate 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 ap- 

 parent sensory endowments are entirely dependent upon its connection with 

 the posterior column of the spinal cord, through the posterior roots. 



125. 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. 



f" 

 V. Nature of the Changes in the Nervous System. 



126. Of the actual nature of the changes, by which impressions are received 

 upon the peripheral origins of the afferent nerves, or are communicated to the 

 central origins of the motor, and are conducted along each to their opposite 

 extremities, physiologists have no certain knowledge. That they are electri- 

 cal has been, and still continues to be, a favourite theory with some ; and that 



