134 Mr. Le Gros Clark on the Mechanism of Respiration. 



3. The results due to this passive tension are that (a) it retains the sup- 

 plemental air in the lungs ; (b) it limits the encroachment of the abdo- 

 minal viscera on the thoracic cavity ; (e) by virtue of the attachment of 

 the pericardium to the cordiform tendon, the uniform calibre of this bag is 

 secured, and the heart is thus protected from being impeded in its move- 

 ments during respiration, the crura taking part in maintaining this condi- 

 tion when the muscle contracts ; (d) it economizes active power in in- 

 spiration. 



4. Inspiration. — When the upper ribs are fixed by the scaleni, both sets 

 of intercostal muscles act in increasing the transverse diameter of the 

 chest, by raising the curve of the ribs and the sternum. 



They rotate the ribs outwards. 



They fix the thoracic walls, and thus antagonize the tendency of the 

 diaphragm to draw inwards the ribs to which it is attached. A fixed cir- 

 cumference is thereby secured, from which the diaphragm acts in altering 

 its own form ; and this is one of the most important functions of the in- 

 tercostal muscles. 



They assist in expiration when the lowest ribs are fixed by the ab- 

 dominal muscles. 



5. The scalenus anticus and posticus (especially the former) are auxilia- 

 ries in inspiration by raising and fixing the first and second ribs, and thus 

 rendering them relatively immovable. 



Thus, when the scaleni act, the intercostal muscles raise the ribs; when 

 the scaleni are at rest, and the abdominal muscles act, the intercostals de- 

 press the ribs ; if neither the scaleni nor abdominal muscles were to act, 

 the ribs would be approximated at their centre by the action of the inter- 

 costal muscles. 



6. The pectoralis minor (when the coracoid process is fixed), the lower 

 costal portion of the pectoralis major, and some other muscles of minor 

 importance might assist in elevating the ribs ; but it is questionable whether 

 they ever do so in such way as to assist in inspiration. 



7. The serratus magnus has no action in inspiration ; all but its lowest 

 digitation must draw the ribs downwards, if they act on them at all. 



8. The action which the sterno-mastoid is capable of exerting in inspi- 

 ration is by fixing the first rib through the medium of the clavicle, and by 

 raising the sternum. This is not required in health, but may be witnessed 

 occasionally in disease. 



9. Expiration. — Ordinary expiration is accomplished by the elastic re- 

 siliency of the lungs, the tense diaphragm resuming its arched form 

 when the muscle ceases to contract : the elasticity of the ribs and ab- 

 dominal parietes may assist to a limited extent ; after a deep inspiration 

 this elasticity has a more important share in expiration. The abdominal 

 muscles, which, conjointly with the levator ani, are the agents of forced 

 expiration, do not act by urging the abdominal viscera against the tense 

 diaphragm, which would resist the pressure, to the injury of these viscera, 



