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Page 6
The various conditions which may act on an organism producing the
changes which are necessary for disease are manifold. Lack of
resistance to injury, incapacity for adaptation, whether it be due to
a congenital defect or to an acquired condition, is not in itself a
disease, but the disease is produced by the action on such an
individual of external conditions which may be nothing more than those
to which the individuals of the species are constantly subject and
which produce no harm.
[Illustration: FIG. 3.--A SECTION OF THE SKIN. 1. A hair. Notice there
is a deep depression of the surface to form a small bulb from which
the hair grows. 2. The superficial or horny layer of the skin; the
cells here are joined to form a dense, smooth, compact layer
impervious to moisture. 3. The lower layer of cells. In this layer new
cells are continually being formed to supply those which as thin
scales are cast off from the surface. 4. Section of a small vein. 9.
Section of an artery. 8. Section of a lymphatic. The magnification is
too low to show the smaller blood vessels. 5. One of the glands
alongside of the hair which furnishes an oily secretion. 6. A sweat
gland. 7. The fat of the skin. Notice that hair, hair glands and sweat
glands are continuous with the surface and represent a downward
extension of this. All the tissue below 2 and 3 is the corium from
which leather is made.]
[Illustration: FIG. 4.--DIAGRAMMATIC SECTION OF A SURFACE SHOWING THE
RELATION OF GLANDS TO THE SURFACE. (_a_) Simple or tubular gland,
(_b_) compound or racemose gland.]
All of the causes of disease act on the body from without, and it is
important to understand the relations which the body of a highly
developed organism such as man has with the world external to him.
This relation is effected by means of the various surfaces of the
body. On the outside is the skin [Fig. 3], which surface is many times
increased by the existence of glands and such appendages to the skin
as the hair and nails. A gland, however complicated its structure, is
nothing more than an extension of the surface into the tissue beneath
[Fig. 4]. In the course of embryonic development all glands are formed
by an ingrowth of the surface. The cells which line the gland surface
undergo a differentiation in structure which enables them to perform
certain definite functions, to take up substances from the same source
of supply and transform them. The largest gland on the external
surface of the body is the mammary gland [Fig. 5] in which milk is
produced; there are two million small, tubular glands, the sweat
glands, which produce a watery fluid which serves the purpose of
cooling the body by evaporation; there are glands at the openings of
the hairs which produce a fatty secretion which lubricates the hair
and prevents drying, and many others.
[Illustration: FIG. 5.--A SECTION OF THE MAMMARY GLAND. (_a_) The
ducts of the gland, by which the milk secreted by the cells which line
all the small openings, is conveyed to the nipple. All these openings
are continuous with the surface of the skin. On each side of the large
ducts is a vein filled with blood corpuscles.]
[Illustration: FIG. 6.--PHOTOGRAPH OF A SECTION OF THE LUNG OF A MOUSE.
_x x_ are the air tubes or bronchi which communicate with all of
the small spaces. On the walls of the partitions there is a close
network of blood vessels which are separated from the air in the
spaces by a thin membrane.]
The external surface passes into the interior of the body forming two
surfaces, one of which, the intestinal canal, communicates in two
places, at the mouth and anus, with the external surface; and the
other, the genito-urinary surface, which communicates with the
external surface at one place only. The surface of the intestinal
canal is much greater in extent than the surface on the exterior, and
finds enormous extensions in the lungs and in the great glands such as
the liver and pancreas, which communicate with it by means of their
ducts. The extent of surface within the lungs is estimated at
ninety-eight square yards, which is due to the extensive infoldings of
the surface [Fig 6], just as a large surface of thin cloth can, by
folding, be compressed into a small space. The intestinal canal from
the mouth to the anus is thirty feet long, the circumference varies
greatly, but an average circumference of three inches may safely be
assumed, which would give between seven and eight square feet of
surface, this being many times multiplied by adding the surfaces of
the glands which are connected with it. A diagram of the microscopic
structure of the intestinal wall shows how little appreciation of the
extent of surface the examination with the naked eye gives [Fig. 7].
By means of the intestinal canal food or substances necessary to
provide the energy which the living tissue transforms are introduced.
This food is liquefied and so altered by the action of the various
fluids formed in the glands of the intestine and poured out on the
surface, that it can pass into the interior of the body and become
available for the living cells. Various food residues representing
either excess of material or material incapable of digestion remain in
the intestine, and after undergoing various changes, putrefactive in
character, pass from the anus as feces.
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