548 MUSCLES OF EXPIRATION. [BOOK n. 



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. 



>j 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 in- 

 going air that any active movements 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 inwards 

 at the same time. Thus, synchronous with the respiratory expan- 

 sion and contraction of the chest, and the respiratory elevation 

 and depression of the alas nasi, there is a rhythmic widening and 

 narrowing of the glottis. Like the movements of the nostril, this 

 respiratory action of the glottis is much more evident in laboured 

 than in tranquil breathing. Indeed in the latter case it is 

 frequently absent. The manner in which this rhythmic opening 



