574 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



considerable amount of air remains in the lungs, and this quantity is 

 spoken of as the residual volume. 



2. THE MECHANICAL PROCESSES OF RESPIRATION. The Mechanism of 

 Inspiration. Every increase in the diameters of the thorax produces, as 

 a consequence, for the reasons already referred to, an expansion of the 

 lungs ; hence, air enters the lungs from the difference in atmospheric 

 pressures. Such a movement is termed an inspiration. It is clear that 

 in the production of inspiration the natural elasticity of the lungs and 

 the thoracic walls must be overcome. Inspiration is, therefore, an active 

 movement and requires the exertion of muscular force. The diameters 

 of the thorax may be increased either through the elevation of the ribs 

 or through the descent of the diaphragm. It is through the latter that 

 in quiet respiration inspiration is produced. The diaphragm may, there- 

 fore, be regarded as the principal muscle 

 of inspiration. In its condition of relax- 

 ation the muscular fibres of the diaphragm 

 together form a curved surface whose con- 

 cavity extends far up into the thorax. 

 When the muscular fibres of the diaphragm 

 shorten they tend to form a straight line 

 between their origins and insertions, and, 

 therefore, the diaphragm in its condition 

 of extreme contraction tends to form an 

 almost plane surface across the lower 

 P 01> tion of the thorax. The origins and 

 THE DIAPHRAGM. (Xtciard.) insertions of the muscular fibres of the 



If A represent a plane extending in expi- 

 ration from the sternum to the vertebra, and diaphragm may be regarded as compara- 



D the position of the diaphragm, in inspiration J 



^n p deL e end W to 1 r ovet tt ' whilethediaphragm tivel y fixed. The central tendon is, 



moreover, but slightly movable, since it is 



firmly connected with the organs occupying the mediastinum above, and 

 below is more or less supported by the liver and stomach. It is, there- 

 fore, evident that the diaphragm in inspiration cannot become completely 

 flattened, since its curvature corresponds with that of the curvature of 

 the abdominal organs. When the diaphragm, then, contracts, the curve 

 is slightly flattened out, and this muscle may, therefore, be regarded as 

 acting as a curved piston which descends in the cavity of the thorax, and 

 so increases its long diameter ; at the moment in which the diaphragm 

 contracts, the ribs in which it is inserted anteriorly are actively elevated. 

 Thus, while the diaphragm in descending tends to lengthen the vertical 

 diameter of the chest, the elevation of the inferior ribs would appear to 

 diminish this diameter. 



The ascent of the lower ribs is, nevertheless, much less extensive 

 than the descent of the diaphragm. It is further to be noticed that every 



