RESPIRATORY MOVEMENTS. 



373 



by virtue of the elasticity of these structures the contents of the 

 abdomen return to the position they occupied at the beginning of 

 the diaphragm's descent, and in so doing this structure is carried 

 back to its original position. The elevation of the ribs by the 

 contraction of the external intercostals during inspiration twists 

 the elastic costal cartilages which join the ribs to the sternum : as 

 soon as these muscles cease to contract these cartilages untwist, 

 and in so doing aid in the return of the ribs. In describing the 

 structure of the lungs it was stated that the walls of the lobules 

 are rich in elastic tissue : in inspiration these lobules are greatly 

 distended, their walls being put on the stretch. When the inspira- 

 tory forces cease to act, then this tissue, by virtue of its elasticity-, 

 returns to its former condition, and in so doing expels the air, 

 constituting expiration. Contractility may be said to be the 

 inspiratory force ; elasticity, the expiratory force. 



As in inspiration, so in expiration, there are occasions when 

 obstruction to the outgoing air exists, and forced expiration be- 

 comes necessary. The muscles concerned in this act are known 



FIG. 210. The larynx in gentle 

 breathing : L, epiglottis ; V, vocal cords ; 

 S, cartilages of Santorini, which sur- 

 mount the arytenoid cartilages ; P, P, 

 ventricular bands (Lennox-Browne). 



FIG. 211. The larynx in deep breath- 

 ing : W, P, tracheal rings ; P., openings 

 of bronchi; P, P, ventricular bands 

 (Lennox-Browne) . 



as extraordinary muscles of expiration or muscles of forced expira- 

 tion, whose arrangement is such that in their contraction the 

 capacity of the thorax is diminished. They have been already 

 described (p. 371). The abdominal walls, by exerting pressure 

 on the abdominal viscera, and thus on the diaphragm, still further 

 diminish the thoracic cavity and force out the contained air. 



Movements of the. Glottis. There are in connection 

 with the process of respiration certain movements of the glottis 

 which are important. On examination of the interior of the 

 larynx it will be seen that during inspiration the vocal cords 

 separate, and during expiration approach each other. During 

 deep breathing (Fig. 211) the separation of the cords is greater 

 than in quiet breathing (Figs. 210, 219). 



The area of the trachea is nearly three times that of the space 

 between the cords at the beginning of inspiration. The separation 

 of these cords is effected by'the contraction of the posterior cricp- 

 arytenoid muscles, which, by their attachment to the arytenoid 

 cartilages, rotate these outward, and thus separate the posterior 



