318 FUNCTIONS OF THE NERVOUS SYSTEM. 



410. The functions of the Pharyngeal and Laryngeal branches of the Pneu- 

 mogastric having been already explained ( 378, 379, and 385), we may now 

 proceed to its Pulmonary division. In regard to this, we have to notice, that 

 its endowments are chiefly afferent ; its most important office being, to con- 

 vey to the Medulla Oblongata the impression produced by venous blood in 

 the capillaries of the lungs, or of carbonic acid in the air-cells. This impres- 

 sion may give rise, as we have seen, to respiratory movements, without pro- 

 ducing sensation; but if it be from any cause stronger than usual, the sense of 

 uneasiness which it occasions is very distressing. The impression may be 

 imitated by pressure on the nerve ; which occasions an immediate inspiratory 

 movement. Hence the -chief function of the afferent portion of the pulmonary 

 division of the Par Vagum, is to serve as an excitor to the respiratory move- 

 ments ; which are consequently diminished in frequency, when the trunk is 

 divided on both sides. But this division also contains motor fibres, which 

 are distributed upon the muscular fibres surrounding the bronchial tubes ; and 

 the experiments of Dr. Williams, which have been recently confirmed by 

 Longet and Volkmann, agree in proving, that the calibre of the bronchial tubes 

 can be caused to contract in a very considerable degree, by stimuli applied to 

 this nerve, and especially by electricity. 



411. Various alterations are produced in the Lungs, by section of the Pneu- 

 mogastric nerves. The order in which these arise, and the causes to which 

 they are immediately due, constitute very interesting subjects of investigation; 

 and the knowledge of them will probably throw light upon many ill-understood 

 morbid phenomena. 



a. 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 immediate 

 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. Never- 

 theless, 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 experimented 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 conges- 

 tion 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 portions 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 puriform effusion into 

 the small bronchial tubes; and in two, gangrene had supervened. 



b. 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 pulmo- 

 nary vessels and the laboured respiration or whether the effusion is the effect of a pre- 

 viously congested state of the blood-vessels. The former is the opinion of many physiolo- 

 gists, 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 froihy scrum 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 dilliculty of respiration, and still further to impede the cir- 

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



