74 ZOOLOGY. 



atmn>pliere. To expel the air from the lungs, it is only 

 necessary that this muscular action should cease for an in- 

 stant; a forcible expiration is effected by means of a volun- 

 tary ellbrt. and is only used occasionally. Respiration is 

 wholly an instinctive action. 



To understand its mechanism, so simple in its results, so 

 complex in the machinery, it is first necessary to examine tin- 

 structure of the thorax. 



This cavity (Fig. 50), has the form of a conoid, with the 

 summit upwards and the base downwards, and its walls form 

 a kind of cage, with an osseous basis composed of the ribs, 

 the sternum, and a portion of the vertebral column. The 

 spaces left between the ribs are filled with the intercostal 

 (internal and external) muscles; the scaleni pass from the 

 cervical vertebrae to the first and second ribs ; powerful mus- 

 cles also convey from the shoulder and arm-bones to the ribs, 

 thus contributing in every way to enable the animal to act 

 powerfully during inspiration, when the great muscular 

 efforts of the body are made ; and in addition, and that the 

 most important of all, the abdominal wall of the thorax is 

 formed chiefly by the diaphragm, the great muscle of respira- 

 tion. 



139. The dilatation of the chest may be effected in two 

 ways by the contraction of the diaphragm or by the eleva- 

 tion of the ribs. 



In repose (expiration) the diaphragm forms an arch towards 

 the chest; in action (inspiration) it contracts and desivnd> 

 towards the abdomen, pushing the contents of the abdomen 

 before it. Thus the capacity of the chest is enlarged, and as 

 the vacuum thus formed in the lungs is gradually being 

 established, the external air rushes in to fill up the spaee. 



The play of the ribs is rather more complex. These osseous 

 arches, extending from the vertebral column to the sternum 

 (with the exception of the lower ones), and articulated with 

 both, are much lower anteriorly than posteriorly, and thus 

 admit of elevation and depression. As the ribs are raised, 

 they rotate on themselves, and thus the cavity of the chest 

 becomes enlarged in all directions. They raise the sternum 

 with them. 



140. In expiration, the diaphragm is relaxed; the 

 external air acts on the walls of the chest; the elastieity oi 

 the lungs assists, and thus the air is expelled from the lungs; 

 the intercostal muscles also cease to play. But in forcible 



