FUNCTIONS OF THE PAR VAGT7M. 319 



been overlooked by others that this frothy fluid is not mucus, though occasionally mixed 

 with it; but that it is the frothy serum so frequently found in cases where the circulation 

 through the lungs has been impeded before death. From this and other facts, Dr. R. con- 

 cludes " that the congestion of the blood-vessels is the first departure from the healthy state 

 of tin- lung, and that the effusion of frothy serum is a subsequent effect." 



c. The next point, therefore, to be inquired into, is the cause of this congestion ; and this 

 is most satisfactorily explained, upon the general principles regulating the circulation of the 

 blood, by remembering that section of the Par Vagum greatly diminishes the frequency of 

 the respiratory movements, and that the quantity of air introduced into the lungs is, there- 

 fore, very insufficient for the due aeration of the blood. We shall hereafter see reason to 

 regard it as one of the best established principles in Physiology, that the activity of the 

 changes which the blood undergoes in the capillary vessels, does, in some way or other, 

 regulate its movement through them ; that, when these changes are proceeding with ac- 

 tivity, the capillary circulation is proportionably accelerated ; and that when they are ab- 

 normally low in degree, the movement of the blood in the capillaries is stagnated. There 

 is now abundant evidence, in regard to the Pulmonary circulation in particular, that, to pre- 

 vent the admission of oxygen in the lungs, either by causing the animal to breathe pure 

 nitrogen or hydrogen, or by occlusion of the air-passages, is to bring the circulation through 

 their capillaries to a speedy check. Hence we should at once be led to infer, that diminu- 

 tion in the number of Respiratory movements would produce the same effect; and as little 

 or no difference in their frequency is produced by section of one Vagus only, the usual ab- 

 sence of morbid changes in the lung supplied by it is fully accounted for. The congestion 

 of the vessels, induced by insufficient aeration, satisfactorily accounts not only for the effusion of 

 serum, but also for the tendency to pass into the inflammatory condition, sometimes pre- 

 sented by the lungs, as by other organs similarly affected. Dr. Reid confirms this view, by 

 the particulars of cases of disease in the human subject, in which the lungs presented after 

 death a condition similar to that observed in the lower animals after section of the Vagi ; 

 and in these individuals, the respiratory movements had been much less frequent than natu- 

 ral during the latter part of life, owing to a torpid condition of the nervous centres. The 

 opinion (held especially by Dr. Wilson Philip) that section of the par vagum produces the 

 serous effusion, by its direct influence on the function of Secretion, is further invalidated by 

 the fact stated by Dr. Reid, that he always found the bronchial membrane covered with its 

 true mucus, except when inflammation was present. 



" The experimental history of the Par Vagum," it is justly remarked by 

 Dr. Reid, "furnishes an excellent illustration of the numerous difficulties with 

 which the physiologist has to contend, from the impossibility of insulating 

 any individual organ from its mutual actions and reactions, when he wishes 

 to examine the order and dependence of its phenomena." In such investi- 

 gations, no useful inference can be drawn from one or two experiments only ; 

 in order to avoid all sources of fallacy, a large number must be made ; the 

 points in which all agree must be separated from others, in which there is a 

 variation of results ; and it must be then inquired, to what the latter is due. 



412. These observations apply equally to the other principal subject of in- 

 quiry in regard to the functions of the Par Vagum, its influence upon the 

 process of Digestion. The results obtained by different experimenters have 

 led to differences of opinion as to its action, no less remarkable than those 

 which have prevailed on the question just discussed. Thus, in regard to the 

 afferent fibres of the Gastric division of the nerve, some physiologists main- 

 tain it to be by impressions on them alone, that the sense of hunger or satiety 

 is excited; whilst others deny that they have any power of transmitting such 

 impressions, which, according to them, do not originate in the stomach at all. 

 Dr. Reid has arrived at the conclusion, from his numerous experiments, that 

 the Par Vagum is the channel through which the mind becomes cognizant of 

 the condition of the stomach ; but that it is not the sole excitor of the sense 

 of hunger. Animals, which have sustained section of the nerve on both sides, 

 will eagerly take food, if they have not received too great a shock from the 

 operation ; but they seem to experience no feeling of satiety when the sto- 

 mach is loaded. This inference is confirmed by Valentin, who mentions that 

 puppies after the operation will take three times the same quantity of milk, 

 as uninjured individuals of the same age, so as greatly to distend the abdomen. 



