520 OF RESPIRATION. 



densation was so great, that considerable portions of the lungs sank in water, 

 and did not crepitate ; but they did not present the granulated appearance of 

 the second stage of ordinary pneumonia. In five cases in which the animals 

 had survived a considerable time portions of the lungs exhibited the second, and 

 even the third stages of pneumonia, with puriform effusion in the small bron- 

 chial tubes ] and in two, gangrene had supervened. One of the most important 

 points to ascertain in an investigation of this kind, is the first departure from a 

 liealthy state ; to decide whether the effusion of frothy reddish serum, by inter- 

 fering with the usual change in the lungs, causes the congested state of the 

 pulmonary vessels and the labored respiration ; or whether the effusion is the 

 effect of a previously congested state of the bloodvessels. The former is the 

 opinion of many physiologists, who have represented the effusion of serum as a 

 process of morbid secretion, directly resulting from the disorder of that function 

 produced by the section of the nerve ; the latter appears the unavoidable infer- 

 ence from the carefully noted results of Dr. Reid's experiments. In several of 

 these, only a very small quantity of frothy serum was found in the air-tubes, 

 even when the lungs were found loaded with blood, and when the respiration 

 before death was very labored. This naturally leads us to doubt, whether the 

 frothy serum is the cause of the labored respiration, and of the congested state 

 of the pulmonary vessels, in those cases where it is present ; though there can 

 be no doubt that, when once it is effused, it must powerfully tend to increase 

 the difficulty of respiration, and still further to impede the circulation through 

 the lungs. Dr. R. has satisfied himself of an important point which has been 

 overlooked by others, namely, that this frothy fluid is not mucus, though occa- 

 sionally 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. concludes " that the congestion of the bloodvessels 

 is the first departure from the healthy state of the lung, and that the effusion 

 of frothy serum is a subsequent effect." The next point, therefore, to be in- 

 quired into, is the cause of this congestion ; and this is most satisfactorily ex- 

 plained, in accordance with the general laws of the Circulation ( 527), by re- 

 membering that section of the Pneumogastrics greatly diminishes the frequency 

 of the respiratory movements, and that the quantity of air introduced into the 

 lungs is, therefore, very insufficient for the due aeration of the blood. There is 

 now abundant evidence, in regard to the Pulmonary circulation in particular, 

 that, to prevent 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 ( 575). 

 Hence we should at once be led to infer, that diminution in the number of Re- 

 spiratory 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 con- 

 gestion 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 inflam- 

 matory condition, 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 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 func- 

 tion 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 



