224 MOUTH-BREATHING AND NASAL BREATHING. 



impairing the expansion of the lungs), or only the expiratory pressure may fall 

 (in cases of emphysema and of asthma) , or both may be weakened (as occurs in 

 feeble persons) . 



If a forced inspiration rarefies the air in the air-passages, the trachea and 

 bronchi become narrowed and shortened; the reverse occurs during expiration. 



If a lung be inflated, air will steadily escape through the walls of the alveoli 

 and trachea. The same thing takes pla'ce during violent expiratory efforts (cuta- 

 neous emphysema attending whooping-cough) , so that pneumothorax, entrance of 

 air into the blood-vessels, and even death may result. 



If a dog be made to breathe through Muller's valve, by means of which the 

 resistance to respiration may be increased at will, it is found that a pressure of 

 40 cm. of water is still readily overcome, that a higher pressure can be overcome 

 for a short time, and one of 70 cm. not at all. 



Until birth the airless lungs lie collapsed (atelectatic) in the chest-cavity, and 

 fill it, so that pneumothorax is not produced if the thorax be opened in a dead 

 fetus. Even in children that have lived for eight days and have breathed normally, 

 the lungs do not collapse when the pleural cavity is opened, but remain in contact 

 with the chest-wall. It is only after further growth that the thorax becomes so 

 large that the lungs must expand under elastic tension; only then will opening 

 of the thorax cause the lungs to contract into a smaller volume. Hermann calls 

 attention to the fact that a lung containing air cannot be emptied by pressure 

 from without. The reason for this is that the small bronchi will be closed by 

 the pressure before the air can leave the alveoli. The muscles of expiration, 

 therefore, have not the power to compress the lungs until they are airless; but, 

 on the other hand, the inspiratory muscular power is sufficient to expand the 

 lungs beyond the state of elastic equilibrium. Hence, the physical attributes of 

 the lungs limit, to a certain extent, the mechanism of respiration: the muscles of 

 inspiration expand the lungs and at the same time increase their elastic tension, 

 while the expiratory muscles can only diminish the tension, without being able 

 to abolish it altogether. 



MOUTH-BREATHING AND NASAL BREATHING. 



Quiet respiration is usually performed with the mouth closed, pro- 

 vided the nose be unobstructed. The current of air passes through the 

 naso-pharyngeal cavity, and there undergoes certain changes: (i) Its 

 temperature is increased to the extent of -f of the difference between 

 its original temperature and that of the body. (2) At this increased 

 temperature it is saturated with aqueous vapor. These changes are 

 made so that the cold, dry air does not irritate the lining of the lungs. 

 (3) Dust-particles may cling to the mucus covering the irregular walls 

 of the air-passages, and are again conveyed outward by the ciliated 

 epithelium. The nasal secretion possesses qualities harmful to many 

 bacteria (for example, anthrax-bacilli), thus demonstrating the salutary 

 effect of nasal breathing when there is danger of contagion. (4) Finally, 

 by means of the sense of smell bad air and air impregnated with injurious 

 admixtures can be recognized. When the mouth is open no current of 

 air passes through the nose during respiration. 



Pathological. Permanent obstruction of the nose, leading to exclusive mouth- 

 breathing, may result in a long series of harmful effects; namely, catarrhal condi- 

 tions of the pharynx, the air-passages, and the middle ear, abnormal formations 

 in the bones of the mouth and the nose, pains in the facial muscles, changes in 

 speech, disturbances of intellect (difficulty in fixing the attention). 



Another important phenomenon is the appearance of edema of the lungs; 

 that is, an exudation of serum from the blood into the pulmonary alveoli. The 

 causes of this condition are: (i) marked obstruction to circulation in the aortic 

 system; for example, after ligation of all of the carotid arteries, or of the arch 

 of the aorta in such a position that only one carotid remains pervious; (2) occlusion 

 of the pulmonary veins; (3) cessation of action in the left ventricle (following 

 mechanical injury), while the right ventricle still continues to beat. All of these 

 causes will produce at the same time anemia of the brain, resulting in anemic 



