68 RESPIRATION. 



a column of blood two metres in height, the work done 

 by the left ventricle in each systole would amount to about 

 four-tenths of a kilogramme- meter, without counting any 

 work done within the heart itself. 



RESPIRATION. 



The alternating in-flow and out-flow of air, which con- 

 stitute respiration, result from the action of muscles which, 

 by changing the capacity of the chest, produce corre- 

 sponding, though not necessarily proportional, variations 

 of the capacity of the thoracic air cavity. In the state of 

 rest, that is when the chest is not acted on by contracting 

 muscles, its capacity is determined by the opposed trac- 

 tions of elastic structures in a state of tension namely, 

 that of the lungs, which tends to diminish it, and those of 

 the ribs and cartilages, intercostal muscles, and diaphragm, 

 which tend to enlarge it. The capacity which the chest 

 possesses under this condition is called the capacity of 

 equilibrium. In ordinary tranquil breathing the chest is 

 expanded beyond its equilibrium capacity in inspiration, 

 but returns to it in expiration. The muscles by which 

 this is effected are the diaphragm and the scaleni, which 

 act by increasing the vertical diameter of the chest, and 

 the external intercostals, levatores, and intercartilaginous 

 internal intercostals, which increase its girth. When a 

 larger exchange of air is required by the organism 

 than can be thus secured, other inspiratory muscles 

 come into play, which, by their combined action, aid in 

 the expansion of the chest in inspiration, while in expir- 

 ation the whole visceral cavity is constricted by the 

 action of the muscles of the abdominal wall, of the lower 

 internal intercostals, of the serrati postici inferiores, and 

 of the sacro-lumbales in consequence of which action the 

 chest acquires in expiration a capacity less than that of 

 equilibrium. 



