174 FUNCTIONS OF THE NERVOUS SYSTEM. 



investigation; and the knowledge of them will probably throw light upon 

 many ill-understood morbid phenomena. In the first place, it has been fully 

 established by Dr. Reid, that section of the Vagus on one side only does not 

 necessarily, or even generally, induce disease of that lung; and hence the 

 important inference may be drawn, that the nerve does not exercise any imme- 

 diate influence on its functions. When both Vagi are divided, however, the 

 animal rarely survives long ; but its death frequently results from the disorder 

 of the digestive functions. Nevertheless, the power of digestion is sometimes 

 restored sufficiently to re-invigorate the animals ; and their lives may then be 

 prolonged for a considerable time. In fifteen out of seventeen animals experi- 

 mented on by Dr. Reid, the lungs were found more or less unfit for the 

 healthy performance of their functions. The most common morbid changes 

 were a congested state of the blood-vessels, and an effusion of frothy serum 

 into the air-cells and bronchial tubes. In eight out of the fifteen, these changes 

 were strongly marked. In some portions of the lungs, the quantity of blood 

 was so great as to render them dense. The degree of congestion varied in 

 different parts of the same lung ; but it was generally greatest at the most 

 depending portions. The condensation was generally greater than could be 

 accounted for by the mere congestion of blood in the vessels ; and probably 

 arose from the escape of the solid parts of the blood into the tissue of the 

 lung. In some instances, the condensation was so great that considerable por- 

 tions of the lung 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 puri- 

 form effusion into the small bronchial tubes ; and, in two, gangrene had super- 

 vened. 



232. One of the most important points to ascertain, in an investigation of 

 this kind, is the first departure from a healthy state, to decide whether the 

 effusion of frothy reddish serum, by interfering with the usual change in the 

 lungs, causes the congested state of the pulmonary vessels and the laboured 

 respiration ; or whether the effusion is the effect of a previously congested 

 state of the blood-vessels. 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 inference 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 laboured. This naturally leads us to doubt whether the frothy 

 serum is the cause of the laboured 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 cir- 

 culation through the lungs. Dr. R. has satisfied himself of an important 

 point, which has 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. concludes "that the 

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

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



233. The next point, therefore, to be inquired into, is the cause of this con- 

 gestion; 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 



