THE RESPIRATORY SYSTEM. 399 
like respiration, which in the nature of the case must be in 
the higher groups of vertebrates ceaseless, and it is not surpris- 
ing that, like a lame dog, which prefers progression by three 
legs to none at all, the ever-active center will keep up its rhythm 
as long as it can—perfectly, if possible, and, if not perfectly, as 
well as it can. Wemean to imply that its action must be 
rhythmic, or cease entirely. 
THE EFFECTS OF VARIATIONS IN THE ATMOSPHERIC 
PRESSURE. 
These depend in great part upon the suddenness with which 
the change is made. When an individual ascends a high 
mountain or rises in a balloon, parts in contact with the air 
become reddened and swollen, owing to the distention of the 
small vessels, which may result in hemorrhages. There is dif- 
ficulty in breathing, the respirations become more rapid, as also 
the pulse. If the lowering of pressure amounts to from one 
third to one half, the quantity of oxygen in the blood is dimin- 
ished, and the carbon dioxide imperfectly excreted. Owing to 
the excess of blood in the superficial parts, the internal organs 
become anzmic, and there is consequently diminished secretion 
of urine and a variety of other disturbances, with general weak- 
ness. The blood-pressure is also altered. 
Sudden diminution of pressure gives rise to a liberation of 
gas—chiefly nitrogen—within the blood-vessels, which causes 
death by blocking the circulation in the small vessels (hence 
also the danger from section of a large vein in surgical opera- 
tions about the neck, the air being liable to be sucked in, owing 
to the negative pressure). 
Increase in the atmospheric pressure when not very great 
gives rise to symptoms akin to those of narcotic poisoning; 
but when the increase amounts to twenty atmospheres, animals 
die, as if asphyxiated, with convulsions. Neither the assump- 
tion of oxygen nor the separation of carbon dioxide takes place 
to the usual extent; and it is interesting to note that micro- 
organisms are killed under similar circumstances. 
With considerable diminution of pressure, though not suf- 
ficient to lead to a fatal result, symptoms the opposite of those 
described above occur. Thus, there is paleness of the surface, 
respiration is easy, the capacity of the lungs is increased, owing, 
it is thought, to the greater descent of the diaphragm, in con- 
sequence of the compression of the gases of the intestines. 
