RESPIRATION. 573 



When the exchange of O and CO 2 is interfered with for a long period, as 

 in cases of prolonged labor, the respiratory centre may become so depressed 

 that spontaneous respirations do not occur upon the birth of the child. In 

 such a case respirations may usually be initiated by irritation of the skin, as 

 by slapping, sprinkling with iced water, etc. Respirations may also be carried 

 on successfully by artificial means (see p. 553). 



In utero the lungs are devoid of air ; the sides of the alveoli and of the 

 small air-passages are in apposition, although the lungs completely fill the 

 compressed thoracic cavity. During the first inspiration comparatively little 

 air is taken into the lungs, because of the force necessary to overcome the 

 adhesion of the sides of the alveoli and of the smaller air-tubes, but as one 

 inspiration follows another inflation increases more and more until full disten- 

 tion is accomplished. The vigorous crying which so generally occurs immedi- 

 ately after birth doubtless is of value in facilitating this expansion. If once 

 the lungs have been filled with air, they are never completely emptied of it, 

 either by volitional effort or by collapse after excision. 



M. THE INNERVATION OP THE LUNGS. 



The nerves of the lungs are derived from the pneumogastrics, the sympa- 

 thetics, and the upper dorsal nerves. Scattered along the paths of distribution 

 of these fibres are many small ganglia. 



The Pneumogastric Nerves. The pulmonary branches of the pneumogas- 

 tric nerves contain not only fibres which convey impulses that affect the gen- 

 eral characters of the respiratory movements, but other fibres that are of 

 great importance to the respiratory mechanism. Setting aside the effects on 

 the respiratory movements following section and stimulation of one or of both 

 vagi, there are observed phenomena which are of an entirely different character, 

 and which are due to excitation or paralysis of certain other specific nerve- 

 fibres. Among these fibres are efferent and afferent broncho-constrictors and 

 broncho-dilators. Roy and Brown 1 found in investigations upon dogs that 

 stimulation of one vagus caused constriction of the bronchi in both lungs; 

 section of one vagus was followed by expansion of the bronchi in the corre- 

 sponding lung, which expansion was sometimes preceded by a slight contraction 

 owing to the temporary irritation caused by the section ; stimulation of the 

 peripheral end of the cut nerve caused a contraction of the bronchi in both 

 lungs ; stimulation of the central end of the cut nerve was followed by a con- 

 traction of the bronchi in both lungs, but not so marked as when the peripheral 

 end was stimulated ; stimulation of sensory nerves other than the vagus rarely, 

 and then only to a slight extent, caused contraction ; atropine paralyzed the 

 constrictor fibres ; nicotine in small doses had a powerful expansive effect on 

 the bronchi ; after etherization stimulation of either the central or the periph- 

 eral end of the cut pneumogastric nerve was often followed by broncho-dilata- 



1 Journal of Physiology, vol. 6, 1885 (Proceedings of the Physiological Society, iii. p. xxi.) ; 

 Einthoven, Pfliiger's Archiv fur Physiologic, 1892, vol. 51, p. 367 ; Sandeman, Du Bois-Reymond'x 

 Arehh fur Physiologie, 1890, p. 252. 



