MANUAL OF PHYSIOLOGY. 



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 upward from the upper part of the thorax 

 to the cervical vertebrae, 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 upward 

 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 frame-work 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 inte- 

 rior of the bellows. When the frame-work enlarges its capacity, 

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

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

 tne expansion of the frame-work. 



By the motions of the frame- work a stream of air passes in or 

 out of the sack ; 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. 



THORACIC MOVEMENTS. 



Physiologically, the motions are divided into two sets (1) 

 those which enlarge the thoracic cavity, and cause the air to 

 rush into the lungs, called inspiration; and (2) those which 



