THE MECHANICS OF PULMONARY RESPIRATION. 343 



tilages cause them to return to their previous position, thus depressing the 

 ribs and diminishing the dimensions of the chest. When 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 reac- 

 tion ; 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 labored, the abdominal muscles become im- 

 portant expiratory agents. By pressing on the contents of the abdomen, 

 they thrust them and therefore the diaphragm also, up toward 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 activity 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 ser- 

 ratus 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. 



279. Faded and laryngeal respiration. The thoracic respiratory move- 

 ments 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 vas- 

 cular 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, probably by the 

 action of the dilatores naris, and thus the entrance of air facilitated. The 

 return to their previous condition during expiration is effected by the elas- 

 ticity of the nasal cartilages, assisted perhaps by the compressors naris. 

 This movement of the nostrils, perceptible in many people even during tran- 

 quil breathing, becomes very obvious in labored respiration. 



When the mouth is closed, the soft palate, which is held somewhat 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 move- 

 ments are met with. When the larynx (the details of which we shall have 

 to deal with at a later part of this work) is examined with the laryngoscope, 

 it is frequently seen that, while during inspiration the glottis is widely open, 

 with each expiration the arytenoid cartilages approach each other so as to 

 narrow the glottis, the cartilages of Santorini projecting inward at the same 

 time. Thus, synchronous with the respiratory expansion and contraction of 

 the chest, and the respiratory elevation and depression of the alse nasi, there 

 is a rhythmic widening and narrowing of the glottis. Like the movements 



