at minus 7.5 mm. Hg at the end of a quiet inspiration. 

 During forced inspiration the value may reach minus 40 mm. 

 Hg. The pressure in the pleural cavity i. r., outside of 

 the lungs and within the thorax is known as intrathofadc 

 pressure, while that within the lungs and respiratory passages 

 is called the intrapulmonary pressure. The variations in 

 pressure are caused by changes in the capacity of the thorax, 

 which may enlarge in all directions. It is obvious that when 

 the thorax increases in size the decreased pressure within 

 allows the entrance of air from the outside, where it is at a 

 higher pressure. The air rushes through the trachea and 

 inflates the lungs. This constitutes an inspiration. The oppo- 

 site process, or an expulsion of air by a decrease in the size 

 of the thorax, is expiration, and both together form respiration. 

 The lungs during respiration are entirely passive, and merely 

 follow the thoracic walls because the atmospheric pressure 

 acting on their inner surfaces is greater than that between 

 the lungs and the thoracic walls. The pleurae are moistened 

 with lymph, and slide over each other without friction. 



Inspiration is an active process brought about by certain 

 muscles which, by their action, enlarge the thorax in a vertical, 

 anteroposterior, and lateral direction. The upper part of 

 the thoracic cage being fixed, the vertical diameter is increased 

 by the descent of the diaphragm in contracting. Other muscles 

 act on the ribs, with the result that 'their sternal ends are 

 raised up and carried forward, enlarging the anteroposterior 

 diameter; at the same time the direction of the axes of rota- 

 tion of the ribs causes them to rotate outward and upward, 

 so increasing the lateral diameter. The chief muscles of inspira- 

 tion are the diaphragm, the scaleni, the serrati postici superiores 

 et inferiores, the levatores costarum breves et longi, and the 

 external intercostals and interchondrals. The diaphragm 

 projects into the thoracic cavity in the form of a flattened 

 dome. During contraction it descends from 5.5 to 11.5 mm. 

 in quiet respiration, and about 42 mm. in deep inspiration. 

 There is a tendency for the diaphragm to pull in its points 

 of attachments the lower ribs, with their cartilages, and the 

 lower portion of the sternum, but usually this is counter- 

 balanced by the pressure of the abdominal viscera. The 

 serrati postici inferiores assist the diaphragm by fixing the 



