PRESSURE CHANGES IN THE RESPIRATORY PASSAGES 321 



taken in at each inspiration the greatest part therefore reaches the alveoli. 

 It is evident that this noxious space must exercise a greater influence, the 

 more superficial the respiration. 



6. CONCOMITANT RESPIRATORY MOVEMENTS 



Besides the muscles already considered as influencing the capacity of the 

 chest, still others are active at the same time, which are of some importance 

 for normal respiration. 



Among these are the muscles which move the vocal cords. In quiet respi- 

 ration the glottis is rather widely open and makes but slight movements 

 (Czermak). But in more active respiration it is widened by contraction of 

 the posterior cricoarytenoid muscles at each inspiration. When the muscles 

 of the vocal cords are paralyzed, the cords take an oblique position with their 

 upper surfaces directed outward. Their inner edges are thus separated but 

 slightly from one another and, being relaxed, are drawn toward each other 

 by the current of air. In young animals complete closure of the glottis may 

 be produced in this way and suffocation be the result (Le Gallois). 



7. SPECIAL FORMS OF RESPIRATORY MOVEMENTS 



The following are to be mentioned as special forms of respiratory move- 

 ments: (1) Coughing, a powerful expiration produced reflexly and begun with 

 a closed glottis, which is then opened by an explosive blast of air under high 

 pressure, whence the accompanying sound; (2) Sneezing, a powerful reflex expi- 

 ration, with open glottis and the mouth cavity closed off from the pharynx: it 

 is often introduced by a deep inspiration; (3) Laughing, a series of short and 

 weak expiratory blasts with lightly closed glottis; (4) Yawning, a deep inspira- 

 tion with the glottis widely open, and as a rule with the mouth open; (5) Sigh- 

 ing, a deep inspiration followed by a prolonged expiration with partially closed 

 glottis; (6) Sobbing is distinguished from sighing only by the velocity of the 

 inspiratory act; it is usually accompanied by a spasmodic ascent of the larynx. 

 All these forms of respiratory movements are produced reflexly, or are the accom- 

 paniments of psychical states and are even then to a certain extent reflex. 



8. PRESSURE CHANGES IN THE RESPIRATORY PASSAGES 



On account of the slight force necessary to expand the lungs, when the 

 thorax is enlarged they begin to expand immediately at the beginning of 

 inspiration, and the alveolar air naturally is at first somewhat rarefied. With 

 the glottis open new air flows in from outside; but in view of the relative 

 narrowness of the respiratory passage and the constrictions occurring at dif- 

 ferent places along it, the inflow cannot take place instantly, and as a conse- 

 quence one always finds a negative pressure in the air passages during the 

 inspiratory phase. Vice versa, when expiration takes place, the air to be 

 exhaled cannot escape immediately; hence we always find a positive pressure 

 in the passages during the expiratory phase. 



With an open glottis and a static condition of the thorax, the tension of 



