410 ANIMAL PHYSIOLOGY. 
The law of habit is well illustrated in the case of divers, who 
can bear deprivation of air longer than those unaccustomed to 
such submersion in water. Greater toleration on the part of 
the respiratory center has probably much to do with the case, 
though doubtless many other departures from the normal occur, 
either independently or correlated to the changes in the respira- 
tory center. 
Summary of the Physiology of Respiration—The purpose of 
repiration in all animals is to furnish oxygen for the tissues 
and remove the carbonic anhydride they produce, which in all 
vertebrates is accomplished by the exposure of the blood in 
capillaries to the atmospheric air, either free or dissolved in 
water. A membrane lined with cells always intervenes between 
the capillaries and the air. 
The air may be pumped in and out, or sucked in and forced 
out. 
Respiration in the Mammal.—The air enters the lungs, owing 
to the enlargement of the chest in three directions by the action 
of certain muscles. It leaves the lungs because of their own 
elastic recoil and that of the chest-wall chiefly. Inspiration is 
active, expiration chiefly passive. 
The diaphragm is the principal muscle of respiration. In 
some animals there is a well-marked facial and laryngeal as 
well as thoracic respiration. Respiration is rhythmical, con- 
sisting of inspiration, succeeded without appreciable pause by 
expiration, the latter being in health of only slightly longer 
duration. There is also a definite relation between the number 
of respirations and of heart-beats. According as respiration is 
normal, hurried, labored, or interrupted, we describe it as 
eupnea, hyperpnea, dyspnea, and apnea. The intra-thoracic 
pressure is never equal to the atmospheric—i.e., it is always 
negative—except in forced expiration; and the lungs are never 
collapsed so long as the chest is unopened. The expired air 
differs from that inspired in being of the temperature of the 
body, saturated with moisture, and containing about 4 to 5 
per cent less oxygen and 4 per cent more carbonic anhydride, 
besides certain indifferently known bodies, the result of tissue 
metabolism, excreted by the lungs. 
The quantity of air actually moved by a respiratory act, as 
compared with the total capacity of the respiratory organs, is 
small; hence a great part must be played by diffusion. The 
portion of air that can not be removed from the lungs by any 
respiratory effort is relatively large. 
