PHYSIOLOGICAL ANATOMY.
ABSORPTION.
[Illustration: Fig. 35.
Villi of the small intestine greatly magnified.]
_Absorption_ is the vital function by which nutritive materials are
selected and imbibed for the sustenance of the body. Absorption, like
all other functional processes, employs agents to effect its purposes,
and the _villi_ of the small intestine, with their numberless projecting
organs, are specially employed to imbibe fluid substances; this they do
with a celerity commensurate to the importance and extent of their
duties. They are little vascular prominences of the mucous membrane,
arising from the interior surface of the small intestine. Each villus
has two sets of vessels. (1.) The blood-vessels, which, by their
frequent blending, form a complete net-work beneath the external
epithelium; they unite at the base of the villus, forming a minute vein,
which is one of the sources of the portal vein. (2.) In the center of
the villus is another vessel, with thinner and more transparent walls,
which is the commencement of a lacteal.
The _Lacteals_ originate in the walls of the alimentary canal, are very
numerous in the small intestine, and, passing between the laminae of the
mesentery, they terminate in the _receptaculum chyli_, or reservoir for
the chyle. The mesentery consists of a double layer of cellular and
adipose tissue. It incloses the blood-vessels, lacteals, and nerves of
the small intestine, together with its accessory glands. It is joined to
the posterior abdominal wall by a narrow _root_; anteriorly, it is
attached to the whole length of the small intestine. The lacteals are
known as the absorbents of the intestinal walls, and after digestion is
accomplished, are found to contain a white, milky fluid, called _chyle_.
The chyle does not represent the entire product of digestion, but only
the fatty substances suspended in a serous fluid.
Formerly, it was supposed that the lacteals were the only agents
employed in absorption, but more recent investigations have shown that
the blood-vessels participate equally in the process, and are frequently
the more active and important of the two. Experiments upon living
animals have proved that absorption of poisonous substances occurs, even
when all communication by way of the lacteals and lymphatics is
obstructed, the passage by the blood-vessels alone remaining. The
absorbent power which the blood-vessels of the alimentary canal possess,
is not limited to alimentary substances, but through them, soluble
matters of almost every description are received into the circulation.
The _Lymphatics_ are not less important organs in the process of
absorption. Nearly every part of the body is permeated by a second
series of capillaries, closely interlaced with the blood-vessels,
collectively termed the _Lymphatic System_. Their origin is not known,
but they appear to form a _plexus_ in the tissues, from which their
converging trunks arise. They are composed of minute tubes of delicate
membrane, and from their net-work arrangement they successively unite
and finally terminate in two main trunks, called the _great lymphatic
veins_. The lymphatics, instead of commencing on the intestinal walls,
as do the lacteals, are distributed through most of the vascular tissues
as well as the skin. The lymphatic circulation is not unlike that of the
blood; its circulatory apparatus is, however, more delicate, and its
functions are not so well understood.
[Illustration: Fig. 36.
A general view of the Lymphatic System.]
The _lymph_ which circulates through the lymphatic vessels is an
alkaline fluid composed of a plasma and corpuscles. It may be considered
as blood deprived of its red corpuscles and, diluted with water. Nothing
very definite is known respecting the functions of this fluid. A large
proportion of its constituents is derived from the blood, and the exact
connection of these substances to nutrition is not properly understood.
Some excrementitious matters are supposed to be taken from the tissues
by the lymph and discharged into the blood, to be ultimately removed
from the system. The lymph accordingly exerts an important function by
removing a portion of the decayed tissues from the body.
[Illustration: Fig. 37.
1. A representation of a lymphatic
vessel highly magnified. 2. Lymphatic
valves. 3. A lymphatic gland and its vessels.]
In all animals which possess a lacteal system there is also a lymphatic
system, the one being the complement of the other. The fact that lymph
and chyle are both conveyed into the general current of circulation,
leads to the inference that the lymph, as well as the chyle, aids in the
process of nutrition. The body is continually undergoing change, and
vital action implies waste of tissues, as well as their growth. Those
organs which are the instruments of motion, as the muscles, cannot be
employed without wear and waste of their component parts. Renovated
tissues must replace those which are worn out, and it is a part of the
function of the absorbents to convey nutritive material into the general
circulation. Researches in microscopical anatomy have shown that the
skin contains multitudes of lymphatic vessels and that it is a powerful
absorbent.
Absorption is one of the earliest and most essential functions of animal
and vegetables tissues. The simpler plants consist of only a few cells,
all of which are employed in absorption; but in the flowering plants
this function is performed by the roots. It is accomplished on the same
general principles in animals, yet it presents more modifications and a
greater number of organs than in vegetables. While animals receive their
food into a sac, or bag called the _stomach_, and are provided with
absorbent vessels such as nowhere exist in vegetables, plants plunge
their absorbent organs into the earth, whence they derive nourishing
substances. In the lower order of animals, as in sponges, this function
is performed by contiguous cells, in a manner almost as elementary as in
plants. In none of the invertebrate animals is there any _special_
absorbent system. Internal absorption is classified by some authors as
follows: _interstitial_, _recrementitial_, and _excrementitial_; by
others as _accidental_, _venous_, and _cutaneous_. The general cutaneous
and mucous surfaces exhale, as well as absorb; thus the skin, by means
of its sudoriferous glands, exhales moisture, and is at the same time as
before stated, a powerful absorbent. The mucous surface of the lungs is
continually throwing off carbonic acid and absorbing oxygen; and through
their surface poisons are sometimes taken into the blood. The continual
wear and waste to which living tissues are subject, makes necessary the
provision of such a system of vessels for conveying away the worn-out
materials and supplying the body with new.
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