MECHANISM OF ORDINARY EXPIRATION. 



175 



along with the diaphragm, or whether the contractions of these two muscles alternate. As the 

 result of their experiments, they conclude that "the internal intercostal muscles are expiratory 

 throughout their whole extent, at least in the dog and cat ; and that in the former animal they 

 are almost ' ordinary ' muscles of respiration, while in the latter they are ' extraordinary ' 

 respiratory muscles."] Landois is of 



opinion that the chief action of these ft 



muscles is not to raise or depress the 

 ribs, but rather that the external inter- 

 costals and the intercartilaginei offer 

 resistance to the inspiratory dilatation 

 of the intercostal spaces, and to the 

 simultaneously increased elastic tension 

 of the lungs. The internal intercostals 

 act during powerful expiratory efforts, 

 (e.g., coughing), and oppose the disten- 

 sion 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 

 (?Serratus anticns major) can 

 only act as elevators of the ribs 

 when the shoulders are fixed, partly 

 by the rhomboidei, and partly by 

 fixing the shoulder-joint and sup- 

 porting the arms, as is done instinc- 

 tively by persons suffering from 

 breathlessness. 



(3) Muscles acting on the Ster- 

 num, Clavicle, and Vertebral 

 Column. When the head is fixed by the muscles of the neck, the sternocleidomastoid 

 raises 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 dila- 

 tation of the intercostal spaces which aid inspiration. During deep respiration, the 

 straightening of the vertebral column takes place involuntarily. 



(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 

 inspiratory movement. During inspiration the pharynx becomes narrow (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 inspiration. 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 inspiratory 

 forces cease, the costal cartilages return to their normal position, i.e., the position 



Fig. 139. 

 Scheme of the action of the intercostal muscles. 



