442 THE NERVOUS SYSTEM. 



ascribed to the hindrance to the passage of blood through the 

 lungs, in consequence of the diminished supply of air and the 

 excess of carbonic acid in the air-cells and in the pulmonary 

 capillaries (see p. 187) ; in part, perhaps, to paralysis of the 

 bloodvessels, leading to congestion ; and in part, also, as the 

 experiments of Traube especially show, they appear due to the 

 passage of food and of the various secretions of the mouth and 

 fauces through the glottis, which, being deprived of its sensi- 

 bility, is no longer stimulated or closed in consequence of their 

 contact. He says, that if the trachea be divided and separated 

 from the oesophagus, or if only the oesophagus be tied, so that 

 no food or secretion from above can pass down the trachea, no 

 degeneration of the tissue of the lungs will follow the division 

 of the pneumogastric nerves. So that, on the whole, death 

 after division of the pneumogastric nerves may be ascribed, 

 when it occurs quickly in young animals, to suffocation through 

 mechanical closure of the paralyzed glottis : and, when it occurs 

 more slowly, to the congestion and pneumonia produced by 

 the diminished supply of air, by paralysis of the bloodvessels, 

 and by the passage of foreign fluids into the bronchi ; and ag- 

 gravated by the diminished frequency of respiration, the in- 

 sensibility to the diseased state of the lungs, the diminished 

 aperture of the glottis, and the loss of the due nervous influ- 

 ence upon the process of respiration. 



4. Respecting the influence of the pueumogastric nerves on 

 the movements of the oesophagus and stomach, the secretion 

 of gastric fluid, the sensation of hunger, absorption by the 

 stomach, and the action of the heart, 1 former pages may be 

 referred to. 



Cyon and Ludwig have discovered that a remarkable power 

 appears to be exercised on the dilatation of the bloodvessels 

 by a small nerve, which arises, in the rabbit, from the superior 

 laryngeal branch, or from this and the trunk of the pneumo- 

 gastric nerve, and after communicating with filaments of the 

 inferior cervical ganglion proceeds to the heart. If this nerve 

 be divided, and its upper extremity be stimulated by a weak 

 interrupted current, an inhibitory influence is conveyed to the 

 vaso-motor centre in the medulla oblongata (p. 452), so as to 

 cause, by reflex action, dilatation of the principal bloodvessels, 

 with diminution of the force and frequency of the heart's 

 action. From the remarkable lowering of the blood-pressure 

 in the vessels, thus produced, this branch of the vagus is called 

 the depressor nerve ; and it is presumed, as an afferent nerve 

 of the heart, to be the means of conveying to the vaso-motor 



1 See foot-note, p. 453. 



