THE REGULATION IN HEALTH AND DISEASE 563 



which draws air into the lungs. The respiratory movements of 

 the newly born infant are irregular in frequency and depth, and 

 are often interrupted by periods during which no breath is taken. 

 The average number of respirations cannot, therefore, be stated 

 with exactitude ; it appears to be about 58 per minute, and varies 

 from 40 to 80. Periods of apncea and of waxing and waning 

 respiration, as in Cheyne-Stokes's breathing, are of normal occur- 

 rence in the healthy young infant, especially during sleep ; the 

 regular sequence of respiration is not acquired until infancy is 

 past. The excitability of the respiratory centre appears to be 

 low in both the foetus and the infant. Cohnstein and Zuntz found 

 that the addition of carbon dioxide to the inspired air produced 

 a much less marked increase in the amplitude of the respiration 

 of the newly born animal than in a young animal a few days old. 

 The condition of the foetal blood and its relation to foetal apncea 

 will be considered in the next chapter. 



What are the factors which maintain the rhythm of respira- 

 tion when once it has started ? Are the same factors effective 

 for its maintenance and regulation as for its commencement, or 

 is some other influence at work ? To these questions no decisive 

 answer can be given, for the opinions of physiologists have been 

 as much divided upon this question as upon the cause of the first 

 breath ; here, again, the two rival views have been stimulation 

 of the respiratory centre by chemical changes in the blood and 

 excitation by nervous impulses received from the periphery. The 

 older view, that the respiratory movements are regulated by the 

 gaseous composition of the blood flowing through the medulla 

 oblongata, is well founded and logical, but, since the publication 

 of the important researches of Hering, Breuer, and Head upon 

 the influence of the vagus upon the rhythm of respiration, it has 

 lost favour and has been abandoned by many physiologists. It 

 was replaced by the theory that the respiratory movements are 

 self -regulating reflex actions ; each respiration by a stimulation 

 of the afferent fibres of the vagus nerves causes the centre in the 

 medulla oblongata to send out expiratory impulses, and each 

 expiration in a similar manner causes the following inspiration. 

 The vagus nerve contains two classes of afferent fibres, stimula- 

 tion of the one kind inhibits inspiration and produces expiration, 

 stimulation of the other inhibits expiration and produces inspira- 

 tion. By " positive " ventilation of the lungs of a rabbit a con- 



