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 s.p.a.ces 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 pa.s.s 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 cla.s.sed 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 s.p.a.ces 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 s.p.a.ces. 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 s.p.a.ce. 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 pa.s.sage 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 s.p.a.ces 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 part.i.tion for ill.u.s.trating osmosis.

*Osmosis.*-The term "osmosis" is used to designate the pa.s.sage of liquids through some part.i.tion which separates them. Thus, if a vessel with an upright membranous part.i.tion 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 part.i.tion 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 part.i.tion 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 part.i.tion.

If but one of the liquids penetrates the part.i.tion, the flow will be in but one direction.

3. The liquids on the two sides of the part.i.tion 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 cla.s.s 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 pa.s.s through membranous part.i.tions, while the colloids are not. An example of a colloid is found in the alb.u.min 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 ma.s.ses 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 part.i.tion or membrane.

*Osmosis not a Sufficient Cause.*-The pa.s.sage of materials through animal membranes, according to the principle of osmosis, is limited to crystalloid substances. But colloid substances are also known to pa.s.s through the various part.i.tions of the body. An example of such is found in the proteids of the blood which, as a colloidal solution, pa.s.s through the capillary walls to become a part of the lymph. Perhaps the best explanation offered as yet for this pa.s.sage is that the colloidal substances are changed by the cells lining the capillaries into substances that form true solutions and that after the pa.s.sage 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?