|
Main
- books.jibble.org
My Books
- IRC Hacks
Misc. Articles
- Meaning of Jibble
- M4 Su Doku
- Computer Scrapbooking
- Setting up Java
- Bootable Java
- Cookies in Java
- Dynamic Graphs
- Social Shakespeare
External Links
- Paul Mutton
- Jibble Photo Gallery
- Jibble Forums
- Google Landmarks
- Jibble Shop
- Free Books
- Intershot Ltd
|
books.jibble.org
Previous Page
| Next Page
Page 13
Repair will also take place the more readily the less complicated is
the architectural structure of the part affected. When a series of
tissues variously and closely related to one another enter into the
structure of an organ, there may be new formation of cells; but when
the loss involves more than this, the complicated architectural
structure will not be completely replaced. A brick which has been
knocked out of a building can be easily replaced, but the renewal of
an area of the wall is more difficult. In the kidney, for example, the
destruction of single cells is quickly made good by new cell
formation, but the loss of an area of tissue is never restored. In the
liver, on the other hand, which is of much simpler construction, large
areas of tissue can be newly formed. For the formation of new cells in
a part there must be a sufficient amount of formative material; then
the circulation of the blood becomes more active, more blood being
brought to the part by dilatation of the vessels supplying it.
Repair after a loss can be perfect or imperfect. The tissue lost can
be restored so perfectly that no trace of an injury remains; but when
the loss has been extensive, and in a tissue of complex structure,
complete restoration does not take place and a less perfect tissue is
formed which is called a scar. Examination of the skin in almost
anyone will show some such scars which have resulted from wounds. They
are also found in the internal organs of the body as the result of
injuries which have healed. The scar represents a very imperfect
repair. In the skin, for example, the scar tissue never contains such
complicated apparatus as hair and sweat glands; the white area is
composed of an imperfectly vascularized fibrous tissue which is
covered with a modified epidermis. The scar is less resistant than the
normal tissue, injury takes place more easily in it and heals with
more difficulty.
Loss brought about by the injuries of disease can be compensated for,
even when the healing is imperfect, by increased function of similar
tissue in the body. There always seems to be in the body under the
usual conditions a reserve force, no tissue being worked to its full
capacity. Meltzer has compared the reserve force of the body to the
factor of safety in mechanical construction. A bridge is constructed
to sustain the weight of the usual traffic, but is in addition given
strength to meet unusual and unforeseen demands. The stomach provides
secretion to meet the usual demands of digestion, but can take care of
an unusual amount of food. The work of the heart may be doubled by
severe exertions, and it meets this demand by increased force and
rapidity of contraction; and the same is true of the muscles attached
to the skeleton. The constant exercise of this reserve force breaks
down the adjustment. If the weight of the traffic over the bridge be
constantly all that it can carry, there quickly comes a time when some
slight and unforeseen increase of weight brings disaster. The
conditions in the body are rather better than in the case of the
bridge, because with the increased demand for activity the heart, for
example, becomes larger and stronger, and reserve force rises with the
load to be carried, but the ratio of reserve force is diminished.
This discussion of injury and repair leads to the question of old age.
Old age, as such, should not be discussed in a book on disease, for it
is not a disease; it is just as natural to grow old and to die as it
is to be born. Disease, however, differs in many respects in the old
as compared with the young and renders some discussion of the
condition necessary. Changes are constantly taking place in the body
with the advance of years, and in the embryo with the advance of days.
In every period of life in the child, in the adult, in the middle-aged
and in the old we meet with conditions which were not present at
earlier periods. There is no definite period at which the changes
which we are accustomed to regard as those of old age begin. This is
true of both the external appearances of age and the internal changes.
One individual may be fully as old, as far as is indicated by the
changes of age, at fifty as another at eighty.
With advancing age certain organs of the body atrophy; they become
diminished in size, and the microscopic examination shows absence or
diminished numbers of the cells which are peculiar to them. The most
striking example of this is seen in the sexual glands of females, and,
to a less degree, in those of the male. There is a small mass or
glandular tissue at the root of the neck, the thymus, which gradually
grows from birth and reaches its greatest size at the age of fifteen,
when it begins slowly to atrophy and almost disappears at the age of
forty. This is the gland which in the calf is known as the sweetbread
and is a delicious and valued article of food. The tonsils, which in
the child may be so large as to interfere with breathing and
swallowing, have almost disappeared in the adult; and there are other
such examples.
Previous Page
| Next Page
|
|