Faults of External Respiration. 449 



and constriction of the respiratory passages, diminution of the 

 respiratory surfaces of the Kings, and by any interference with 

 the movements of the respiratory muscles. In conditions of ob- 

 struction of the nasal passages (swelHng of the nasal mucous 

 membrane, tumors), breathing may for the time be still accom- 

 plished through the mouth; obstruction of the larnyx and tra- 

 .chea with its branches, by foreign bodies or tumors in the lumen, 

 spasm of the glottis, external pressure (thyroid tumors, oesopha- 

 geal diverticula, strangulation), or collection of exudates in the 

 tubes, causes difficulty of respiration (dyspna^a, air hunger) in 

 proportion to the extent of the obstruction. Proportionately to 

 the degree of diminution of entrance of air to the lungs the blood 

 becomes impoverished in oxygen and its carbonic acid content 

 increases. Lack of oxygen and excess of carbonic acid in the 

 blood act to stimulate the respiratory centre in the medulla ob- 

 longata (point of exit of the vagus) ; this stimulation, and prob- 

 ably the mechanical influence of incomplete filling of the pulmo- 

 nary alveoli, which directly afifects the fibres of the vagus, bring 

 about an increased contraction of the inspiratory muscles. In- 

 spiration becomes stronger, more powerful, deeper and longer 

 in duration, because the narrowed lumen of ihe tubes does no't 

 admit the external air to pass readily into the alveoli and the de- 

 mand for oxygen in the medulla oblongata is not at once satis- 

 fied (Krehl). The air becomes rarified in the lung and the in- 

 tra-alveolar pressure less than the external pressure upon the 

 outside of the body; and for this reason inspiratory retractions oi 

 the intercostal tissues may be noted. Because the air cannot pass 

 out through the narrowed portions of ^e tract with the usual 

 ease, the lung does not collapse as rapidly as normally and ex- 

 piration is also prolonged (Krehl). 



Contraction of the diaphragm is the main factor of the inspir- 

 atory forces, the contraction of this broad muscular sheet, arch- 

 ing anteriorly . because of the pressure oi the abdominal viscera, 

 causes it to assume a flatter plane and thus increases the thoracic 

 space, the elevation of the chest wall, especially the lower ribs 

 (by the levator costarnm muscles) aiding in producing this ef- 

 fect. Expiration is due to cessation of the diaphragmatic contrac- 

 tion (termination of its tension) and that of the levator muscles, as 

 well as to the elastic contraction of the lung expanded in inspira- 

 tion. Ruptures of the diaphragm (excluding here the question 

 of hcxmorrhage from the ruptured blood vessels and hernia of 



