od is impeded? Give examples. 576. Why do those
persons that have broad chests and voluminous lungs suffer less from
cold than the narrow-chested with small lungs?

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_Observation._ Fishes that breathe by means of gills, as the cod,
pike, &c., depend solely on the small quantity of oxygen that is
contained in the air mixed with the water. Their temperature is not
much greater than the medium in which they live. Whales, dolphins,
&c., breathe by means of lungs, and the inhalation of atmospheric air
makes their temperature about 100 deg., independent of the heat of the
element in which they live.

577. _The quality of respired air influences the generation of animal
heat._ In vestries, and other public rooms, when crowded with an
audience, where the ventilation is inadequate, the lamps will emit but
a faint light, because the oxygen is soon expended, and there is not
enough of the vivifying principle to unite with the oil and disengage
light. In the human body, when the respired air has lost some of its
life-giving properties, the combustion that takes place in different
parts of the system is not so complete as when it contains a proper
proportion of oxygen; and hence less heat is disengaged. For this
reason, those persons that breathe impure air, either in the daytime
or night, require more clothing, than those that work and sleep in
well-ventilated rooms.

578. _The condition of the brain and nervous system affects the
generation of animal heat._ If the brain is diseased, or the mind is
absorbed in thought, depressed by sorrow, or aroused from fear, the
breathing becomes slow and scarcely perceptible, and a chilliness
pervades the body, particularly the extremities; while, on the
contrary, if the mind and nervous system are excited by joyous and
agreeable emotions, the circulation of blood is quicker, and the
system more powerfully resists external cold. During sleep, when the
brain is partially inactive, less heat is generated than when awake.

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What is said of those fishes that breathe by means of gills? Of those
that breathe by means of lungs? 577. Why do lamps give but a faint
light in crowded, unventilated rooms? What effect on animal heat has
impure air? 578. Mention the effects of some of the mental emotions on
animal heat.

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_Observation._ The preceding remark explains why an individual who
sleeps in the same clothing tha