FUNCTIONS OF THE CEPHALIC NERVES. 175 



that the quantity of air introduced into the lungs is, therefore, 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 activity, the capillary circulation is proportionably acce- 

 lerated; and that, when they are abnormally 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 prevent the admis- 

 sion 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 circu- 

 lation through their capillaries to a speedy check. Hence we should at once 

 be led to infer, that diminution 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 absence of morbid changes 

 in the lung supplied by it is fully accounted for. The congestion of the ves- 

 sels, induced by insufficient aeration, satisfactorily accounts, not only for the 

 effusion of serum, but also for the tendency to pass into the inflammatory con- 

 dition, sometimes presented 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 

 natural 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. 



234. " 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 insu- 

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

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

 investigations, 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. 

 These observations apply equally to the other principal subject of inquiry 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 differ- 

 ences of opinion as to its action no less remarkable than those on the question 

 just discussed. Dr. Wilson Philip has long maintained that the par vagum 

 controls the secretion of gastric juice, which he stated to cease when the nerve 

 is divided ; and he further stated, that the influence of galvanism, propagated 

 along the nerve, would re-establish the secretion. This statement has been 

 quoted and re-quoted as an established physiological position ; and, when 

 united with the well-known fact that galvanism would excite muscular con- 

 traction, it has seemed to Dr. W. Philip and other physiologists sufficient to 

 establish the important position, that galvanism and nervous influence are 

 identical. The statement, however, has been disputed by many other experi- 

 menters, who have satisfied themselves that the secretion of gastric juice con- 

 tinued, and that the impairment of the digestive power, which is certainly a 

 result (for a time at least) of the operation, may be attributed to paralysis of 

 the muscular coat of the stomach. 



