CHAP, ii.] RESPIRATION. 577 



simple elastic reaction; but there is probably some, though 

 possibly in most cases, a very slight, expenditure of muscular 

 energy to bring the chest more rapidly to its former condition. 

 This is, as we have seen, supposed by many to be afforded by the 

 internal intercostals acting as depressors of the ribs. If these do 

 not act in this way, we may suppose that the elastic return of the 

 abdominal walls is accompanied and assisted by a contraction of 

 the abdominal muscles. The triangularis sterni, the effect of 

 whose contraction is to pull down the costal cartilages, may also be 

 regarded as an expiratory muscle. 



When expiration becomes laboured, the abdominal muscles 

 become important expiratory agents. By pressing on the contents 

 of the abdomen, they thrust them and therefore the diaphragm 

 also up towards the chest, the vertical diameter of which is 

 thereby lessened, while by pulling down the sternum and the 

 middle and lower ribs they lessen also the cavity of the chest in its 

 antero-posterior and transverse diameters. They are, in fact, the 

 chief expiratory muscles, though they are doubtless assisted by 

 the serratus posticus inferior and portions of the sacro-lumbalis, 

 since when the diaphragm is not contracting, the depression of the 

 lower ribs which the contraction of these muscles causes, serves 

 only to narrow the chest. As expiration becomes more and more 

 forced, every muscle in the body which can either by contracting 

 depress the ribs, or press on the abdominal viscera, or afford fixed 

 support to muscles having those actions, is called into play. 



336. Facial and Laryngeal Respiration. The thoracic 

 respiratory movements are accompanied by associated respiratory 

 movements of other parts of the body, more particularly of the 

 face and of the glottis. 



In normal healthy respiration, the current of air which passes 

 in and out of the lungs, travels, not through the mouth but through 

 the nose, chiefly through the lower nasal meatus. The ingoing air, 

 by exposure to the vascular mucous membrane of the narrow and 

 winding nasal passages, is more efficiently warmed than it would be 

 if it passed through the mouth ; and at the same time the mouth 

 is thereby protected from the desiccating effect of the continual 

 inroad of comparatively dry air. 



During each inspiratory effort the nostrils are expanded, pro- 

 bably by the action of the dilatores naris, and thus the entrance of 

 air facilitated. The return to their previous condition during expi- 

 ration is effected by the elasticity of the nasal cartilages, assisted 

 perhaps by the compressores naris. This movement of the nostrils, 

 perceptible in many people even during tranquil breathing, becomes 

 very obvious in laboured respiration. 



When the mouth is closed, the soft palate which is held some- 

 what tense, is swayed by the respiratory current, but entirely in a 

 passive manner, and it is not until the larynx is reached by the 

 ingoing air that any active movements are met with. When the 



