MECHANISM OF INSPIRATION. 805 



Mechanism of Respiration. 



The respiratory movements are of two kinds, viz. : those which draw 

 air into the breathing organs, or the movements of inspiration, and 

 those which expel the air from those organs, or the movements of ex- 

 piration. A complete respiration therefore consists of an inspiratory 

 and an expiratory act. Inspiration requires a greater effort than ex- 

 piration. At the commencement of the independent existence of air- 

 breathing animals, the former act precedes the latter, the lungs being 

 then filled before air can be expired from them; on the other hand, 

 the final respiratory act consists of an expiration, and to expire is 

 synonymous with to die. 



Inspiration. The thorax is a closed cavity, with movable walls, the 

 available space in which, beyond that occupied by the heart and blood- 

 vessels, oesophagus, thoracic duct, lymphatic glands and nerves, is ac- 

 curately filled by the two lungs. The interior of these spongy organs, 

 however, communicates with the outward atmosphere, through the nose, 

 mouth, pharynx, larynx, trachea, bronchi, and bronchial tubes. Hence 

 any enlargement of the thoracic cavity from expansion of its walls, is 

 immediately accompanied by the entrance of air, through the air- 

 passages just mentioned, into the interior of the lungs, and by the in- 

 flation of those elastic organs, as they follow the expanding walls of 

 the chest. In describing the inspiratory act, it is sometimes said that 

 a virtual vacuum is formed in the thorax, which the air fills by enter- 

 ing through the only passages by which it can reach its interior. But 

 the vacuum in question is only threatened or impending, none being 

 really formed. The equilibrium between the atmospheric pressure on 

 the surface of the lungs, acting through the walls of the thorax, and 

 that on their interior, operating through the open air-passages, being 

 disturbed by the active expansion of the thoracic parietes, through the 

 agency of the inspiratory muscles, the air enters the air-passages sim- 

 ultaneously, in exact and instant correspondence with the amount of 

 expansion, and the lungs as instantly become inflated, and follow the 

 inner surface of the expanding thoracic walls. 



In this inspiratory movement, the thorax is enlarged in each of its 

 three dimensions : in depth from before backwards, in width from 

 side to side, and in length or height from above downwards (see Fig. 

 114). The enlargement in depth, from the spine to the sternum, is 

 accomplished by the elevation of the ribs, which being movably articu- 

 lated with the vertebral column behind, and continued on, by their 

 cartilages, to the sternum in front, and having, moreover, an oblique 

 direction from their posterior to near their anterior extremities, neces- 

 sarily cause an elevation and projection forwards of the sternum, 

 when they are slightly lifted upon their posterior points of attachment, 

 to a less oblique position. The point of support or fulcrum of each 

 rib, is the vertebral column ; whilst the connection of the upper ten 

 ribs in front, directly or indirectly with the sternum, enables them to 

 elevate and push forward that bone, and thus to increase the antero- 

 posterior diameter of the chest. The enlargement of the thorax in 

 width is likewise accomplished by the elevation of the curved and ob- 



