240 NERVOUS SYSTEM. 



" 5. Respiration returns even during excitation, and 

 when this is arrested, it almost always becomes accelerated. 



"6. Arrest in expiration is more easily obtained than 

 arrest in inspiration ; there are animals, indeed, in which it 

 is impossible to effect the latter. 



" 7. If an excitation be employed sufficiently powerful 

 to arrest respiration in inspiration, all -respiratory move- 

 ments may be made to cease at the very moment when the 

 excitation is applied (inspiration, half-inspiration, expira- 

 tion), either by operating on the pneumogastric, or oper- 

 ating upon the laryngeal. . . . 



"Any feeble excitation of centripetal nerves increases 

 the number of the respiratory movements ; any powerful 

 excitation diminishes them. A powerful excitation of the 

 pneumogastrics, of the superior laryngeal, of the nasal 

 branch of the infra-orbital, may arrest them completely ; 

 if the excitation be sufficiently energetic, the arrest takes 

 place at the very moment it is applied. Finally, sudden 

 death of the animal may follow a too powerful impression, 

 thus transmitted to the respiratory centre : all this being 

 true for certain mammalia, birds, and reptiles." 



The above formulated statements express the experimen- 

 tal facts at present known w^ith regard to the influence 

 of the pneumogastrics upon respiration. The pulmonary 

 branches themselves are so deeply situated that they have 

 not as yet been made the subject of direct experiment, with 

 any positive and satisfactory results. A theory has recently 

 been proposed in which it has been assumed that there are 

 two kinds of nerves in the pulmonary branches of the pneu- 

 mogastrics, one set being excited by inflation of the lungs, 

 which excitation gives rise to expiration, the other set being 

 stimulated by collapse of the lungs, which excites inspira- 

 tion ; but the experiments upon which this idea is based are 

 vague and unsatisfactory. 1 



1 HERING, Die Selbststeu rung der Athmimg durch den Nervus vagus. Sitzungs- 

 bericlite der mathematisch-naturuisscnschaftlichen Classe der k. Akademie der Wis* 

 mnschaften, Wien, 1868, Bd. Ivii., 2 Abtheilung, S. 672, et seq. 



