[Fig. 27]
Fig. 27--*Diagram showing position of the lymph* with reference to the
blood and the cells. The central tube is a capillary. The arrows indicate
the direction of slight movements in the lymph.
The blood, it will be remembered, moves everywhere through the body in a
system of _closed_ tubes. These keep it from coming in contact with any of
the cells of the body except those lining the tubes themselves. The
capillaries, to be sure, bring the blood very near the cells of the
different tissues; still, there is need of a liquid to fill the space
between the capillaries and the cells and to transfer materials from one
to the other. The lymph occupies this position and does this work. The
position of the lymph with reference to the capillaries and the cells is
shown in Fig. 27.
*Origin of the Lymph.*--The chief source of the lymph is the plasma of the
blood. As before described, the walls of the capillaries consist of a
single layer of flat cells placed edge to edge. Partly on account of the
pressure upon the blood and partly on account of the natural tendency of
liquids to pass through animal membranes, a considerable portion of the
plasma penetrates the thin walls and enters the spaces occupied by the
lymph.
The cells themselves also help to form the lymph, since the water and
wastes leaving the cells add to its bulk. These mix with the plasma from
the blood, forming the resultant liquid which is the lymph. A considerable
amount of the material absorbed from the food canal also enters the lymph
tubes, but this passes into the blood before reaching the cells.
*Composition and Physical Properties of the Lymph.*(26)--As would naturally
be expected, the composition of the lymph is similar to that of the blood.
In fact, nearly all the important constituents of the blood are found in
the lymph, but in different proportions. Food materials for the cells are
present in smaller amounts than in the blood, while impurities from the
cells are in larger amounts. As a rule the red corpuscles are absent from
the lymph, but the white corpuscles are present and in about the same
numbers as in the blood.
The physical properties of the lymph are also similar to those of the
blood. Like the blood, the lymph is denser than water and also coagulates,
but it coagulates more slowly than does the blood. The most noticeable
difference between these liquids is that of color, the lymph being
colorless. This is due to the absence of red corpuscles. The quantity of
lymph is estimated to be considerably greater than that of the blood.
*Lymph Vessels.*--Most of the lymph lies in minute cavities surrounding the
cells and in close relations with the capillaries (Figs. 27 and 30). These
are called _lymph spaces_. Connecting with the lymph spaces on the one
hand, and with certain blood vessels on the other, is a system of tubes
that return the lymph to the blood stream. The smallest of these, and the
ones in greatest abundance, are called _lymphatics_. They consist of
slender, thin-walled tubes, which resemble veins in structure, and, like
the veins, have valves. They differ from veins, however, in being more
uniform in size and in having thinner walls.
[Fig. 28]
Fig. 28--*Diagram of drainage system for the lymph.* 1. Thoracic duct. 2.
Right lymphatic duct. 3. Left subclavian vein. 4. Right subclavian vein.
5. Superior vena cava. 6. Lacteals. 7. Lymphatic glands. The small tubes
connecting with the lymph spaces in all parts of the body are the
lymphatics.
The lymphatics in different places gradually converge toward, and empty
into, the two main lymph tubes of the body. The smaller of these tubes,
called the _right lymphatic duct_, receives the lymph from the lymphatics
in the right arm, the right side of the head, and the region of the right
shoulder. It connects with, and empties its contents into, the right
subclavian vein at the place where it is joined by the right jugular vein
(Fig. 28).
The larger of the lymph tubes is called the _thoracic duct_. This receives
lymph from all parts of the body not drained by the right lymphatic duct,
and empties it into the left subclavian vein. Connection is made with the
subclavian vein on the upper side at the place where it is joined by the
left jugular vein. The thoracic duct has a length of from sixteen to
eighteen inches, and is about as large around as a goose quill. The lower
end terminates in an enlargement in the abdominal cavity, called the
_receptacle of the chyle_. It is provided with valves throughout its
course, in addition to one of considerable size which guards the opening
into the blood vessel.
The lymphatics which join the thoracic duct from the small intestine are
called the _lacteals_ (Fig. 28). These do not differ in structure from the
lymphatics in other parts of the body, but they perform a special work in
absorbing the digested fat (Chapter XI).
*Lymphatic Glands.*--The lymphatic glands, sometimes called lymph nodes,
are small and somewhat rounded bodies situated along the course of the
lymphatic tubes. They vary in size, some of them being an inch or more in
length. The lymph vessels generally open into them on one side and leave
them on the other (Figs. 28 and 30). They are not glands in function, but
are so called because of their having the general form of glands. They
provide favorable conditions for the development of white corpuscles (page
29). They also separate harmful germs and poisonous wastes from the lymph,
thereby preventing their entrance into the blood.
*Relations of the Lymph, the Blood, and the Cells.*--While the blood is
necessary as a carrying, or transporting, agent in the body, the lymph is
necessary for transferring materials from the blood to the cells and _vice
versa_. Serving as a physiological "go between," or medium of exchange,
the lymph enables the blood to minister to the needs of the cells. But the
lymph and the blood, everything considered, can hardly be looked upon as
two separate and distinct liquids. Not only do they supplement each other
in their work and possess striking similarities, but each is made in its
movements to pass into the vessels occupied by the other, so that they are
constantly mixing and mingling. For these and other reasons, they are more
properly regarded as two divisions of a single liquid--one which, by
adapting itself to different purposes,(27) supplies all the conditions of
a nutrient fluid for the cells.
*Movements of the Lymph.*--As compared with the blood, the lymph must be
classed as a quiet liquid. But, as already suggested, it has certain
movements which are necessary to the purposes which it serves. A careful
study shows it to have three well-defined movements as follows:
1. A movement from the capillaries toward the cells.
2. A movement from the cells toward the capillaries.
3. A movement of the entire body of lymph from the lymph spaces into the
lymphatics and along these channels to the ducts through which it enters
the blood.
By the first movement the cells receive their nourishment. By the second
and third movements the lymph, more or less laden with impurities, is
returned to the blood stream. (See Figs. 28 and 30.)
*Causes of the Lymph Movements.*--Let us consider first the movement
through the lymph tubes. No pump, like the heart, is known to be connected
with these tubes and to supply the pressure necessary for moving the
lymph. There are, however, several forces that indirectly aid in its flow.
The most important of these are as follows:
1. _Blood Pressure at the Capillaries._--The plasma which is forced through
the capillary walls by pressure from the heart makes room for itself by
pushing a portion of the lymph out of the lymph spaces. This in turn
presses upon the lymph in the tubes which it enters. In this way pressure
from the heart is transmitted to the lymph, forcing it to move.
2. _Variable Pressure on the Walls of the Lymph Vessels._--Pressure exerted
on the sides of the lymph tubes by contracting muscles tends to close them
up and to push the lymph past the valves, which, by closing, prevent its
return (Fig. 29). Pressure at the surface of the body, provided that it is
variable, also forces the lymph along. The valves in the lymph vessels
serve the same purpose as those in the veins.
[Fig. 29]
Fig. 29--*Diagram* to show how the muscles pump lymph. _A._ Relaxed muscle
beside which is a lymphatic tube. _B._ Same muscle in state of
contraction.
3. _The Inspiratory Force._--When the thoracic cavity is enlarged in
breathing, the unbalanced atmospheric pressure is exerted from all
directions towards the thoracic space. This not only causes the air to
flow into the lungs (Chapter VII), but also causes a movement of the blood
and lymph in such of their tubes as enter this cavity. It will be noted
that both of the large lymph ducts terminate where their contents may be
influenced by the respiratory movements. (See Practical Work.)
*Where the Lymph enters the Blood.*--The fact that the lymph is poured into
the blood at but two places, and these very close to each other, requires
a word of explanation. As a matter of fact, it is impossible for the lymph
to flow into blood vessels at most places on account of the blood
pressure. This would force the blood into the lymph vessels, instead of
allowing the lymph to enter the blood. The lymph can enter only at some
place where the blood pressure is less than the pressure that moves the
lymph. Such a place is found in the thoracic cavity. As already pointed
out (page 54), the blood pressure in the veins entering this cavity
becomes, with each expansion of the chest, negative, i.e., less than the
pressure of the atmosphere on the outside of the body. This, as we have
seen, aids in the flow of the blood into the right auricle. It also aids
in the passage of lymph into the blood vessels. The lymph is said to be
"sucked in," which means that it is forced in by the unbalanced pressure
of the atmosphere.(28) Some advantage is also gained by the lymph duct's
entering the subclavian vein on the upper side and at its union with the
jugular vein. Everything considered, it is found that the lymph flows into
the blood vessels where it can be "drawn in" by the movements of breathing
and where it meets with no opposition from the blood stream itself (Fig.
30).
[Fig. 30]
Fig. 30--*Diagram* showing general movement of lymph from the place of
relatively high pressure at the lymph spaces to the place of relatively
low pressure in the thoracic cavity.
*Lymph Movements at the Cells.*--The double movement of the lymph from the
capillaries toward the cells and from the cells toward the capillaries is
not entirely understood. Blood pressure in the capillaries undoubtedly has
much to do in forcing the plasma through the capillary walls, but this
tends to prevent the movement of the lymph in the opposite direction.
Movements between the blood and the lymph are known to take place in part
according to a general principle, known as _osmosis_, or dialysis.
[Fig. 31]
Fig. 31--*Vessel* with an upright membranous partition for illustrating
osmosis.
*Osmosis.*--The term "osmosis" is used to designate the passage of liquids
through some partition which separates them. Thus, if a vessel with an
upright membranous partition be filled on the one side with pure water and
on the other with water containing salt, an exchange of materials will
take place through the membrane until the same proportion of salt exists
on the two sides (Fig. 31). The cause of osmosis is the motion of the
molecules, or minute particles, that make up the liquid substance. If the
partition were not present, this motion would simply cause a mixing of the
liquids.
*Conditions under which Osmosis occurs.*--Osmosis may be shown by suitable
experiments (see Practical Work) to take place under the following
conditions:
1. The liquids on the two sides of the partition must be _unlike_ either
in density or in composition. Since the effect of the movement is to
reduce the liquids to the same condition, _a difference in density causes
the flow to be greater from the less dense toward the denser liquid_, than
in the opposite direction; while _a difference in composition causes the
substances in solution to move from the place of greater abundance toward
places of less abundance_.
2. The liquids must be capable of wetting, or penetrating, the partition.
If but one of the liquids penetrates the partition, the flow will be in
but one direction.
3. The liquids on the two sides of the partition must readily mix with
each other.
*Osmosis at the Cells.*--In the body osmosis takes place between the blood
and the lymph and between the lymph and the cells, the movements being
through the capillary walls and the membranes inclosing the cells (Fig.
27). Oxygen and food materials, which are found in great abundance in the
blood, are less abundant in the lymph and still less abundant in the
cells. According to the principle of osmosis, the main flow of oxygen and
food is from the capillaries toward the cells. On the other hand, the
wastes are most abundant in the cells where they are formed, less abundant
in the lymph, and least abundant in the blood. Hence the wastes flow from
the cells toward the capillaries.
*Solutions.*--Neither the blood plasma nor the lymph, as already shown, are
simple liquids; but they consist of water and different substances
dissolved in the water. They belong to a class of substances called
_solutions_. The chief point of interest about substances in solution is
that they are very finely divided and that their little particles are free
to move about in the liquid that contains them. Both the motion and the
finely divided condition of the dissolved substances are necessary to the
process of osmosis. All substances, however, that appear to be in solution
are not able to penetrate membranes, or take part in osmosis.
*Kinds of Solutions in the Body.*--The substances in solution in the body
liquids are of two general kinds known as _colloids_ and _crystalloids_.
The crystalloids are able to pass through membranous partitions, while the
colloids are not. An example of a colloid is found in the albumin of an
egg, which is unable to penetrate the membrane which surrounds it.
Examples of crystalloids are found in solutions of salt and sugar in
water. The inability of a colloid to penetrate a membrane is due to the
fact that it does not form a true solution. Its particles (molecules),
instead of being completely separated, still cling together, forming
little masses that are too large to penetrate the membrane. Since,
however, it has the appearance, on being mixed with water, of being
dissolved, it is called a _colloidal solution_. The crystalloid substance,
on the other hand, completely separates in the water and forms a _true
solution_--one which is able to penetrate the partition or membrane.
*Osmosis not a Sufficient Cause.*--The passage of materials through animal
membranes, according to the principle of osmosis, is limited to
crystalloid substances. But colloid substances are also known to pass
through the various partitions of the body. An example of such is found in
the proteids of the blood which, as a colloidal solution, pass through the
capillary walls to become a part of the lymph. Perhaps the best
explanation offered as yet for this passage is that the colloidal
substances are changed by the cells lining the capillaries into substances
that form true solutions and that after the passage they are changed back
again to the colloidal condition.
*Summary.*--Between the cells and the capillaries is a liquid, known as the
lymph, which is similar in composition and physical properties to the
blood. It consists chiefly of escaped plasma. The vessels that contain it
are connected with the system for the circulation of the blood. By adding
new material to the lymph and withdrawing waste material from it, the
blood keeps this liquid in a suitable condition for supplying the needs of
the cells. Supplementing each other in all respects, the blood and the
lymph together form the nutrient cell fluid of the body. The interchange
of material between the blood and the lymph, and the lymph and the cells,
takes place in part according to the principle of osmosis.
*Exercises.*--1. Explain the necessity for the lymph in the body.
2. Compare lymph and water with reference to density, color, and
complexity of composition.
3. Compare lymph and blood with reference to color, composition, and
movement through the body.
4. Show how blood pressure in the capillaries causes a flow of the lymph.
5. Show how contracting muscles cause the lymph to move. Compare with the
effect of muscular contraction upon the blood in the veins.
6. Trace the lymph in its flow from the right hand to where it enters the
blood; from the feet to where it enters the blood.
7. What conditions prevail at the cells to cause a movement of food and
oxygen in one direction and of waste materials in the opposite direction?
8. What part does water play in the exchanges at the cells?
9. Show that the blood and the lymph together fulfill all the requirements
of a nutrient cell fluid in the body.
PRACTICAL WORK
*To illustrate the Effect of Breathing upon the Flow of Lymph.*--Tightly
holding one end of a glass tube between the lips, let the other end extend
into water in a tumbler on a table. In this position quickly inhale air
through the nostrils, noting that with each inhalation there is a slight
movement of the water up the tube. (No sucking action should be exerted by
the mouth.) Apply to the movements in the large blood and lymph vessels
entering the thoracic cavity.
*To illustrate Osmosis.*--1. Separate the shell from the lining membrane at
one end of an egg, over an area about one inch in diameter. To do this
without injuring the membrane, the shell must first be broken into small
pieces and then picked off with a pair of forceps, or a small knife blade.
Fit a small glass tube, eight or ten inches long, into the other end so
that it will penetrate the membrane and pass down into the yolk. Securely
fasten the tube to the shell by melting beeswax around it, and set the egg
in a small tumbler partly filled with water. Examine in the course of half
an hour. What evidence now exists that the water has passed through the
membrane?
2. Tie over the large end of a "thistle tube" (used by chemists) a thin
animal membrane, such as a piece of the pericardium or a strip of the
membrane from around a sausage. Then fill the bulb and the lower end of
the tube with a concentrated solution of some solid, such as sugar, salt,
or copper sulphate. Suspend in a vessel of water so that the liquid which
it contains is just on a level with the water in the vessel. Examine from
time to time, looking for evidence of a movement in each direction through
the membrane. Why should the movement of the water into the tube be
greater than the movement in the opposite direction? (If the thistle tube
has a very slender stem, it is better to fill the bulb before tying on the
membrane. The opening in the stem may be plugged during the process of
filling.)
[Fig. 32]
Fig. 32--An osmosometer.
NOTE.--With a special piece of apparatus, known as an _osmosometer_, the
principle of osmosis may be more easily illustrated than by the method in
either of the above experiments (Fig. 32). This apparatus may be obtained
from supply houses.