78 ZOOLOGY. 



cells, or cul de sacs, in which the air tubes terminate in mam- 

 mals. In birds, some of these air tubes open into large 

 membranous pouches, which proceed as far as the limbs, and 

 conduct the air even into the interior of the bones. Thus 

 respiration becomes more active in this class of animals. 



138. Mechanism of Respiration in Man. By the 

 movements of inspiration and expiration performed by the 

 walls of the thorax or chest, the air is constantly renewed in, 

 and expelled from, the lungs. The walls of the chest are 

 moveable, and more especially one, which is not seen, the 

 diaphragmatic wall ; dilated by a muscular effort, the air 

 rushes into the cavity through the nostrils and trachea, 

 pressed on by the whole weight of the incumbent atmosphere. 

 To expel the air from the lungs, it is only necessary that this 

 muscular action should cease for an instant ; a forcible expira- 

 tion is effected by means of a voluntary effort, 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 the 

 structure of the thorax. 



This cavity (Fig. 59), 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 

 muscles also proceed 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 being 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 descends 

 towards the abdomen, pushi ig 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 space. 



