656 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



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

 mitting sensory impressions from its peripheral extremity. But not unfre- 

 quently 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. 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 

 exhibited; 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 apparent 

 sensory endowments are entirely dependent upon its connection with the pos- 

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



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

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

 sions, 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 regarded by expe- 

 rimenters. As a general rule, negative results are of less value than positive; 

 but very careful discrimination is often required to determine what are negative, 

 and what positive results. Each particular case has its own sources of fallacy, 

 which require to be logically scrutinized ; and the only satisfactory proof is 

 derived from the concurrence of every kind of evidence, which the nature of 

 the inquiry admits of. Thus, in the determination of the functions of a parti- 

 cular nerve-trunk, it should be shown that a certain effect is constantly produced 

 by its excitation (under the conditions laid down in the preceding paragraph), 

 and that a corresponding interruption in the action to which it is hence inferred 

 to be subservient, takes place when its continuity has been interrupted : by this 

 double proof, the Glosso-pharyngeal and the Pneumogastric are shown to be the 

 principal, but not the sole, exciters of the movements of Deglutition and Inspi- 

 ration respectively. But the evidence afforded solely by the interruption of a 

 particular function, after the division of a certain nerve, or the destruction or 

 removal of a nervous centre, is by no means so satisfactory; for this may be 

 occasioned rather by the general effects of the operation than by the simple 

 lesion of the nervous apparatus. In order to get rid, so far as possible, of this 

 source of fallacy (which particularly affects experiments upon the Encephalic 

 centres, and upon the influence of the nerves upon the viscera), it is desirable 

 to perform comparative experiments, in which the general injury shall be as 

 nearly as possible the same, and the only difference shall lie in the lesion of the 

 nervous system ; and to subtract from the general result all that can be thus 

 shown to be attributable to the general disturbance produced by the operation. 

 But, even then, it may happen that the function is only suspended for a time 

 by the shock which has been induced by the injury to the nerve ; and if it 

 should be subsequently renewed, without any reunion of the trunk, we have the 

 most convincing proof that, whatever degree of participation the nerve may have 

 in it, the action is not essentially dependent upon the integrity of that portion 

 of the nervous apparatus. Such we have seen to be the case in regard to the 

 relation of the Pneumogastric nerves to the secretion of gastric fluid in the walls 

 of the stomach ( 445-447). 



694. All our positive knowledge of the functions of the Nervous System in 

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

 within ourselves, and that which we obtain from watching the manifestations of 



