286 PHYSIOLOGICAL PHYSICS. [Chap. xxm. 



in size of the lungs are produced by variations of 

 pressure in the cavity of the chest. 



The variations may be shortly expressed as diminu- 

 tion of pressure on inspiration, and increase of pressure 

 on expiration. They will produce effects on other 

 thoracic organs. Notably will they affect the circula 

 tion of the blood. For the diminution will aid the 

 flow from the large veins into the heart, while it will 

 interfere with the outward now from the heart into 

 the arteries, the result being favourable to the venous 

 circulation. 



What the constant effects of atmospheric pressure 

 are, becomes very apparent when one ascends a consider- 

 able distance from the sea-level, either by means of a 

 balloon or by climbing a high mountain. The pressure 

 gradually diminishes as one ascends, and the air 

 becomes rarefied. The first effects are quickening of 

 the respirations, because, the air being rarefied, less 

 oxygen is taken in with every inspiration, and to get 

 the ordinary amount more frequent inspiration is 

 necessary. The heart's action is also increased. If 

 the ascent be continued a sense of fatigue is experienced, 

 dyspnoea and venous congestions occur ; and, owing 

 to the pressure from within remaining constant, while 

 the external pressure is greatly reduced, the thin walls 

 of the capillaries may give way, and haemorrhage take 

 place, especially in situations where, owing to the 

 looseness of the texture, external support to the 

 vessels is least, as in the walls of the lungs, the 

 mucous lining of nose and air-passages, lips, etc. 



Still further, the close apposition of bones connected 

 at the joints is largely effected and maintained by the 

 atmospheric pressure, without the need of muscular 

 effort. The brothers Weber showed this by cutting 

 all the muscles and ligaments surrounding the coxo- 

 femoral articulation and the capsule of the joint, but 

 the head of the femur still remained closely applied to 



