xni EESPIEATOEY EHYTHM 503 



stimuli acting on the centres, can and must co-operate with the 

 functional intermittence characteristic of these centres, owing to 

 the alternations of groups and pauses. 



The other extreme cases of periodic respiration are the 

 experiments of Sokolow and Luchsinger, and of Langendorff and 

 Siebert on the frog, and more particularly those so admirably 

 carried out by Fano on the tortoise. In these cases the reflex 

 excitability of the centres (particularly the capacity of reacting to 

 the stimulus of the blood) is not merely lessened, but is entirely 

 suspended. Here, then, periodic respiration is a more simple 

 phenomenon : when reflex excitability is suspended, the automatic 

 excitability of the centres dominates the stage completely, and the 

 grouping of the respirations is the external expression or record of 

 the special mode in which the energy accumulated within them 

 in consequence of the slow processes of metabolism is liberated 

 and developed, until it is finally exhausted. The importance of 

 the facts adduced by Fano consists not in any refutation of 

 the theory formulated by us in 1879, which is left untouched 

 because it deals with an essentially distinct order of phenomena ; 

 but in its demonstration that the two forms of excitability with 

 which the elements of the respiratory centres are endowed do not 

 suffer from the vicissitudes of metabolism in the same degree, 

 since in poikilothermic animals, under certain special conditions, 

 reflex activity may be entirely suspended, or profoundly depressed, 

 while automatic excitability persists and is manifested by char- 

 acteristic upward and downward fluctuations. 



This lengthy chapter has been exclusively devoted to the 

 nervous factors of the respiratory mechanism. But a further 

 series of important facts shows that the chemical respiratory 

 activity of the tissues is also dominated by the nervous system, 

 which may excite or moderate it, and even cause the value of the 

 respiratory quotient, i.e. the ratio between the oxygen absorbed 

 and the carbonic acid given off, to oscillate. 



We shall consider this interesting subject, which exceeds the 

 limits of the physiology of the respiratory apparatus, in due time, 

 along with the metabolism, or material exchanges, of the body as a 

 whole. 



BIBLIOGRAPHY 



The following monographs and memoirs are most frequently quoted from the 

 copious literature of this subject : 

 FLOURENS. Recherches experimentales sur les proprietes et les fonctions du 



systeme nerveux. Paris, 1842. 

 BROWN-SEQUARD. Journal de physiologie, i., 1858. 

 M. SCHIFF. Lehrbuch der Physiol., 1858-59. Gesam. Beitrage zur Physiol., i. 



Lausanne, 1894. 

 ROSENTHAL. Die Atembewegungen und ihre Beziehungen r zum Nervus vagus. 



Berlin, 1892. Hermann's Handbuch d. Physiol., iv. Leipzig, 1882. 

 BERING and BREUER. Sitzungsber. d. Wiener Akademie, Ivii., ii., 1868. 



