232 THE VITAL FUNCTIONS. 



respiration. First, at tlie lower part, or that which cor- 

 responds to the hasis of the cone, the only side, indeed, 

 which is not defended by bone, there is extended a thin ex- 

 pansion, partly muscular, and partly tendinous, forming a 

 complete partition, and closing the cavity of the chest on the 

 side next to the abdomen. This muscle is called the Dia- 

 phragm: it is perforated, close to its origin from the spine, 

 by four tubes, namely, the oesophagus, the aorta, the vena 

 cava, and the thoracic duct. Its surface is not flat, but con- 

 vex above, or towards the chest; and the direction of its 

 fibres is such, that, when they contract, they bring down the 

 middle part, which is tendinous, and render it more flat than 

 before, (the passage of the four tubes already mentioned, not 

 interfering with this action,) and thus, the cavity of the tho- 

 rax may be considerably enlarged. It is obvious that if, 

 upon the descent of the diaphragm, the lungs were to re- 

 main in their original situation, an empty space would be 

 left between them and the diaphragm. But no vacuum can 

 take place in the body; the air cells of the lungs must al- 

 ways contain, even in their most compressed state, a certain 

 quantity of air: and this air will tend, by its elasticity, to ex- 

 pand the cells; the lungs will, consequently, be dilated, and 

 will continue to fill the chest; and the external air will rush 

 in through the trachea in order to restore the equilibrium. 

 This action is termed inspiration. The air is again thrown 

 out when the diaphragm is relaxed, and pushed upwards, by 

 the action of the large muscles of the trunk; the elasticity 

 of the sides of the chest, concurring also in the same effect; 

 and thus expiration is accomplished. 



The muscles which move the ribs conspire also to produce 

 dilatations and contractions of the cavity of the chest. Each 

 rib is capable of a small degree of motion on that extremity 

 by which it is attached to the spine; and this motion, as- 

 suming the chest to be in the erect position, as in man, is 

 chiefly upwards and downwards. But, since the inclination 

 of the ribs is such that their lower edges form acute angles 

 with the spine, they bend downwards as" they proceed to- 

 wards the breast; and the uppermost rib being a fixed point. 



