MECHANISM OF ORDINARY EXPIRATION. 239 



action of these muscles is not to raise or depress the ribs, but rather that the 

 external intercostals and the intercartilaginei offer resistance to the inspiratory 

 dilatation of the intercostal spaces and to the simultaneously increased elastic ten- 

 sion of the lungs. Internal intercostals act during powerful expiratory efforts, 

 (e.g., coughing), and oppose the distension of the lungs and chest caused by this 

 act. Unless muscles were present to resist the uninterrupted tension and pressure, 

 the intercostal substance would become so distended that respiration would be 

 impossible. [According to Rutherford, the internal intercostals are probably 

 muscles of inspiration.] 



The Pectoralis Minor and (1 Serratus Anticus Major) can only act 

 as elevators of the ribs, when the shoulders are fixed, partly by the rhom- 

 boidei, and partly by fixing the shoulder- joint and supporting the arms, 

 as is done instinctively by persons suffering from breathlessness. 



(3.) Muscles acting upon the Sternum, Clavicle and Vertebral Column. 

 When the head is fixed by the muscles of the neck, the sternocleido- 

 mastoid can raise the manubrium sterni, and the sternal end of the 

 clavicle, so that the thorax is raised and thereby dilated. The scaleni 

 also aid in this act. The clavicular portion of the trapezius may 

 act in a similar, although less energetic, manner. When the vertebral 

 column is straightened, it causes an elevation of the upper ribs, and a 

 dilatation of the intercostal spaces which aid inspiration. During deep 

 respiration, this straightening of the vertebral column takes place in- 

 voluntarily. 



(4.) Laryngeal Movements. During laboured respiration, with every 

 inspiration, the larynx descends and the glottis is opened. At the 

 same time the palate is raised, so as to permit a free passage to the 

 air entering through the mouth. 



(5.) Facial Movements. During laboured respiration, the facial 

 muscles are involved ; there is an inspiratory dilatation of the nostrils 

 (well marked in the horse and rabbit.) When the need for respiration 

 is very great, the mouth is gradually widened, and the person as it were 

 gasps for breath. During expiration, the muscles that are active during 

 4 and 5 relax, so that a position of equilibrium is established without 

 there being any active expiratory movement to counteract the inspira- 

 tory movement. During inspiration the pharynx becomes narrower 

 (Garland.) 



(B.) Expiration. Ordinary expiration occurs without the aid of 

 muscles, owing to the weight of the chest, which tends to fall into its 

 normal position from the position to which it was raised during inspira- 

 tion. This is aided by the elasticity of the various parts of the chest. 

 When the costal cartilages are raised, which is accompanied by a slight 

 rotation of their lower margins from below forwards and upwards 

 their elasticity is called into play. As soon, therefore, as the inspira- 

 tory forces cease, the costal cartilages return to their normal position 



