THE PNEUMOGASTRICc 561 



ing sluggishness. There are no convulsions, nor any evidences of pain. 

 After death the lungs are found in a peculiar state of solidification. 

 They are not swollen, but rather appear smaller than natural. They 

 are of a dark purple color, leathery, and resisting to the touch, destitute 

 of crepitation, and infiltrated with blood. Pieces of the lung cut out 

 sink in water. The pleural surfaces, at the same time, are bright and 

 polished, and their cavity contains no effusion or exudation. The lungs 

 are simply engorged with blood, and, to a greater or less extent, empty 

 of air; their tissue having undergone no other alteration. 



These phenomena point to the pneumogastric nerves as the main 

 channels through which the stimulus which excites the movements of 

 respiration is conveyed inward to the medulla oblongata. Respiration 

 is a reflex act, consisting, like other nervous manifestations of a similar 

 character, of two different elements; namely, first, an impression con- 

 veyed from without inward by a sensitive nerve to the appropriate 

 nervous centre; and, secondly, of a motor impulse transmitted thence 

 through motor nerves to the muscular apparatus. But by dividing the 

 pneumogastric nerves in the neck, neither the intercostal muscles nor 

 the diaphragm are paralyzed. The muscular apparatus which effects 

 the expansion of the lungs remains untouched, and yet the movements 

 of respiration become gradually slower until they cease altogether. At 

 the same time the disturbance of respiration, under these circumstances, 

 although sufficient to produce death after a short interval, is not accom- 

 panied by any apparent sense of suffocation. The retarded breathing, 

 and the consequent imperfect aeration of the blood, are not felt by the 

 animal, and he accordingly makes no attempt to compensate for them 

 by voluntary effort. 



In dividing the pneumogastric nerves, therefore, it is not the motor, 

 but the sensitive element in the reflex act of respiration which is inter- 

 fered with. The experiments of Waller and Prevost 1 show conclusively 

 that this is the part performed by the nerves in question. In these 

 experiments the pneumogastric nerve was exposed in the living dog, 

 and divided in its course down the neck; after which, galvanization of 

 its central extremity produced a succession of forcible inspirations and 

 expirations, expelling the air through the trachea with an audible sound. 

 The respiratory impulse, therefore, is propagated through the pneu- 

 mogastric nerve in a centripetal, not in a centrifugal direction. The 

 impression which normally originates in the lungs, and is thence con- 

 veyed through these nerves to the medulla oblongata, produces in the 

 nervous centre, though unperceived as a conscious sensation, the stimu- 

 lus which calls into activity the muscles of respiration. If this impres- 

 sion be not at once satisfied by filling the lungs with air, it increases in 

 intensity ; and if the breath be voluntarily suspended or forcibly ob- 

 structed, the impression soon becomes perceptible as a sensation of 

 distress, or " demand for breath," which reacts upon the entire system. 



1 Archives de Physiologic normale et pathologique. Paris, 1870, p. 190. 



