CONSTRUCTION OF THORAX. 325 



ceeding rib from above downwards increases in length, in the 

 amount of its slope downwards and forwards, and in the obliquity 

 of its shaft. 



The floor of the thorax is formed by a dome-shaped muscle 

 the diaphragm, which bulges with its convex side into the cavity, 

 and separates the thoracic from the abdominal viscera. The 

 upper outlet is closed around the trachea by several muscles, 

 which pass obliquely upwards from the first rib to the cervical 

 vertebrse, and hold the upper part of the thorax in position. 

 These muscles can also elevate as well as fix the first rib, as will 

 be seen when speaking of the muscles in detail. The intervals 

 between the ribs are filled up by two sets of muscle fibres, which 

 cross one another at right angles, and are attached to the margins 

 of the neighboring ribs. 



The base of the thorax is connected by a number of strong 

 muscles with the pelvis and the spine, whence they pass upwards 

 to the lower ribs. The anterior muscles pull down the sternum 

 and anterior part of the ribs. The posterior fix and extend the 

 last rib. 



From a mechanical point of view the thorax may be regarded 

 as a specially arranged bellows, the dimensions of which may be 

 increased in all directions. 



Within the framework of the bellows is an elastic bag, with 

 the interior of which the outer air communicates by an air-pipe, 

 which is the only passage between the atmosphere and the in- 

 terior of the bellows. When the framework enlarges its capacity 

 the pressure of the atmosphere pushes a stream of air into the 

 elastic sac so as to distend it, and thus fill the space caused by 

 the expansion of the framework. 



By the motions of the framework a stream of air passes in or 

 out of the sac ; a small quantity of the air contained in the lungs 

 is thus changed at each breath, and a certain standard of purity 

 kept up. 



In order to fully understand the motions by which the thorax 

 is enlarged, much more detailed knowledge of the anatomy of the 

 bony case and its muscles must be gained than can possibly be 

 given here. 



