nd the body for its protection; to furnish
nutriment for the system when food cannot be taken; to supply the
carbon and hydrogen necessary to sustain the generation of heat, when
these articles of combustion are not otherwise furnished. The
_med'ul-la-ry_ substance, (marrow,) in the cavities of the long bones,
is very much like fat.

_Observation._ During sickness, if there is not emaciation or
absorption of this secretion, it is considered an unfavorable symptom,
because it indicates a want of power in the absorbing system, which is
among the last to be affected.

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How is it proved that secretion depends on nervous influence? 424.
What is said of the secretions from the serous membrane? 425. From
what tissue is a serous fluid exhaled? What is the effect when this
fluid becomes excessive in quantity? What is fat? Its use? What is
marrow?

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426. The mucous secretion is a transparent, viscid fluid which is
secreted by those membranes that line the cavities of the body, which
have an external communication, as the trachea and alimentary canal.
This secretion serves to protect these parts from the influence of the
air, and concurs, by means of its peculiar properties, in the
performance of their functions. 427. There are two external
secretions, namely, one from the skin, called perspiration, and the
other from the lungs. The cutaneous exhalation, or transpiration[14]
exists in two forms, called sensible perspiration (sweat) and
insensible perspiration. The pulmonary exhalation is the most
important and universal, and closely resembles that of the skin.

[14] _Transpiration_ is a term often used generically, to signify the
passage of fluids or gases through membranes, internally or
externally; but _perspiration_ is a specific term, signifying
transpiration on to the external surface.

428. The follicles are found only in the skin and mucous membrane.
They secrete an oily, unctuous substance, which mixes with the
transpiration, and lubricates the skin. At the root of each hair there
is a minute follicle, which secretes the fluid that oils the hair. The
wax in the passage of the ear is secreted from these bodies.

429. All the blood distributed to the different glands is similar in
composition and character; but the fluids secreted by them, vary in
appearance in a remarkable degree. The office of the glands appears to
be principally to for