THE RESPIRATORY SYSTEM. 



373 



with each, expiratory act. This may amount to from 30 to 70 

 millimetres with strong inspiration, and 60 to 100 in forcible ex- 

 piration. 



When inspiration ceases, the elastic recoil of the rib carti- 

 -lages and the ribs themselves, and of the sternum, the weight 

 of these parts and that of. the attached muscles, etc., assists in 

 the return of the chest to its original position, entirely inde- 

 pendently of the action of muscles. Moreover, with the de- 

 scent of the diaphragm the abdominal viscera have been thrust 

 down and compressed together with their included gases; when 

 this muscle relaxes, they naturally exert an upward pressure. 

 Putting these events together, it is not difficult to understand 

 why the air should be squeezed out of the lungs, the elasticity 

 of which latter is, as we have shown, an important factor in 

 itself. 



The Muscles of Eespiration.— The diaphragm may be con- 

 sidered the most important single respiratory muscle, and can 

 of itself maintain respiration. The 

 scaleni are important as fixators 

 of the ribs ; the levatores costa- 

 rum and external intercostals, as 

 normal elevators. The quadra- 

 tus lumborum assists the dia- 

 phragm by fixing the last rib. 

 These, with the aerratus posticus 

 superior, may be regarded as the 

 principal muscles called into ac- 

 tion in an ordinary inspiration. 

 The muscles used in an ordinary 

 expiratory act are the internal in- 

 tercostals, the triungularis stemi, 

 and serratus posticus inferior. 

 In forced inspiration the lower 

 ribs are drawn dovra and re- 

 tracted, giving support in their 

 fixed position to the diaphragm. 

 The scaleni, pectorales, serratus 

 magnus, latissimus dorsi, and oth- 

 ers are called into action ; but 

 when dyspncBa becomes extreme, 

 as in one with a fit of asthma, nearly all the muscles of the 

 body may be called into play, even the muscles of the face. 



Fig. 297.— Laryngoacopic views of 

 the ^ottis, etc. (after Quain and 

 Czermak). I. Larynx in quiet 

 breathing, n. During a deep in- 

 Bpiration. In this case the rings 

 of the trachea and commence- 

 ment of bronchi are visible. 

 Such a condition is persistent in 

 many forms of disease in which 

 respiration is attended with dif- 

 ficulty. 



