438 MUSCLES OF EXPIRATION. [Boon 11. 



the diaphragm descends, in pushing down the abdominal viscera, 

 it puts the abdominal walls on the stretch : and hence, when at 

 the end of inspiration the diaphragm relaxes, the abdominal 

 walls return to their place, and by pressing on the abdominal 

 viscera, push the diaphragm up again into its position of rest. 

 Expiration then during easy breathing is, in the main, 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 inter- 

 nal inter costals 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 contrac- 

 tion 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 con- 

 tents of the abdomen, they thrust them and therefore the dia- 

 phragm 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 ab- 

 dominal viscera, or afford fixed support to muscles having those 

 actions, is called into play. 



267. Facial and Laryngeal Respiration. The thoracic 

 respiratory movements are accompanied by associated respira- 

 tory 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 desic- 

 cating effect of the continual inroad of comparatively dry air. 



During each inspiratory effort the nostrils are expanded, 

 probably by the action of the dilatores naris, and thus the 

 entrance of air facilitated. The return to their previous condi- 

 tion during expiration is effected by the elasticity of the nasal 



