NERVOUS MECHANISM OF RESPIRATION. 289 



diaphragm. The animal thus continues to breathe by a series of pro- 

 longed inspiratory spasms. The centre continues to discharge rhythmic- 

 ally, but its activity is not adequate to the normal aeration of the blood, 

 and the spasms get gradually slower and slower, until the animal finally 

 dies of asphyxia. According to Eosenthal, these imperfect respiratory 

 movements are not altered further by dividing all the posterior roots of 

 the upper cervical nerves, and he concludes, therefore, that the isolated 

 respiratory centre is capable of an automatic rhythmic activity apart 

 from the influence of afferent impulses. 



It must be remembered, in attempting to draw conclusions from this experi- 

 ment, that it is necessarily an imperfect one, since afferent impulses must be 

 ascending to the centre in consequence of stimulation of the cut ends of the 

 nerves as they lie in the wound, and the animal cannot be kept alive suffi- 

 ciently long for the irritating effects of the section to pass off. These im- 

 pulses are constant and not rhythmic in character, so that the only certain 

 conclusion to be drawn from the experiment is that the centre is able to convert 

 constant into rhytbmic discharges. We do not know certainly whether the 

 centre separated from all afferent impulses would discharge at all. H. E. 

 Hering, 1 who has investigated the activity of the spinal cord in the frog after 

 division of all the posterior roots, has concluded that in this case the motor 

 centres are unable to discharge, even under the influence of strychnine, when 

 removed from all peripheral stimuli, and it is jDossible that, were we able to 

 perform a similar experiment on the respiratory centre of mammals, we should 

 meet with like results. 



If we conclude with Rosenthal that the activity of the centre is 

 automatic, we must seek to find out what are, apart from afferent 

 impulses, the local changes in or around the nerve cells by which this 

 activity is evoked. Among the environments of the cell the most 

 important factor in evoking this activity is the condition of the blood 

 circulating through the blood vessels of the part. It has long been 

 known that the activity of the centre is intimately dependent on the 

 condition of the blood, and is especially excited by increased venosity of 

 the blood and allayed by improved arterialisation. Every hindrance to 

 the free renewal of the blood-gases in the lungs calls forth increased 

 respiratory movements, which, if the obstacle be not removed, may 

 involve almost every muscle in the body ; this condition is known as 

 dyspnoea. On the other hand, free renewal of the gases in the blood 

 leads to a condition in which the respiratory movements are rendered 

 shallower or even abolished. This condition, which is known as apncea, 

 is, as we shall see later, most readily produced by reflexes from the 

 lungs, but may also, in the absence of these impulses, be brought about 

 by free aeration of the blood (see p. 301). 



Ever since dyspnoea has been the subject of scientific investigation, 

 the question has been much discussed whether the excitant is to be 

 sought in the increased amount of carbon dioxide, or in the diminished 

 quantity of oxygen in the blood. Opinions at first were divided on this 

 point ; J. Midler 2 and, in his earlier work, Eosenthal, 3 considering the 

 deficiency of oxygen as the stimulus, while Traube 4 and Thiry 5 ascribed 



1 Arch.f. d. ges. Physiol, Bonn, 1893, Bd. liv. S. 614. 



2 Sitzungsb. d. k. Akad. d. Wissensch., Wien, Bd. xxxiii. S. 99 (quoted from Rosenthal). 



3 Loc. cit. (" Die Athembewegungen, etc.," Berlin, 1862). 



4 Ges. Bcitr. z. Path. u. Physiol., Berlin, Bd. i. 



5 Rec. de trav. Soc. med. allcin. dc Paris, 1865 (quoted from Eosenthal). 



VOL II. — 19 



