9. Show that the blood and the lymph together fulfill all the requirements of a nutrient cell fluid in the body.

PRACTICAL WORK

*To ill.u.s.trate the Effect of Breathing upon the Flow of Lymph.*-Tightly holding one end of a gla.s.s 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 ill.u.s.trate Osmosis.*-1. Separate the sh.e.l.l 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 sh.e.l.l must first be broken into small pieces and then picked off with a pair of forceps, or a small knife blade.

Fit a small gla.s.s tube, eight or ten inches long, into the other end so that it will penetrate the membrane and pa.s.s down into the yolk. Securely fasten the tube to the sh.e.l.l 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 pa.s.sed 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 ill.u.s.trated than by the method in either of the above experiments (Fig. 32). This apparatus may be obtained from supply houses.

CHAPTER VII - RESPIRATION

Through the movements of the blood and the lymph, materials entering the body are transported to the cells, and wastes formed at the cells are carried to the organs which remove them from the body. We are now to consider the pa.s.sage of materials from outside the body to the cells and _vice versa_. One substance which the body constantly needs is oxygen, and one which it is constantly throwing off is carbon dioxide. Both of these are const.i.tuents of

*The Atmosphere.*-The atmosphere, or air, completely surrounds the earth as a kind of envelope, and comes in contact with everything upon its surface. It is composed chiefly of oxygen and nitrogen,(29) but it also contains a small per cent of other substances, such as water-vapor, carbon dioxide, and argon. All of the regular const.i.tuents of the atmosphere are gases, and these, as compared with liquids and solids, are very light.

Nevertheless the atmosphere has weight and, on this account, exerts pressure upon everything on the earth. At the sea level, its pressure is nearly fifteen pounds to the square inch. The atmosphere forms an essential part of one's physical environment and serves various purposes.

The process by which gaseous materials are made to pa.s.s between the body and the atmosphere is known as

*Respiration.*-As usually defined, respiration, or breathing, consists of two simple processes-that of taking air into special contrivances in the body, called the lungs, and that of expelling air from the lungs. The first process is known as _inspiration_; the second as _expiration_. We must, however, distinguish between respiration by the lungs, called _external respiration_, and respiration by the cells, called _internal respiration_.

_The purpose of respiration_ is indicated by the changes that take place in the air while it is in the lungs. Air entering the lungs in ordinary breathing parts with about five per cent of itself in the form of oxygen and receives about four and one half per cent of carbon dioxide, considerable water-vapor, and a small amount of other impurities. These changes suggest a twofold purpose for respiration:

1. To obtain from the atmosphere the supply of oxygen needed by the body.

2. To transfer to the atmosphere certain materials (wastes) which must be removed from the body.

The chief organs concerned in the work of respiration are

*The Lungs.*-The lungs consist of two sac-like bodies suspended in the thoracic cavity, and occupying all the s.p.a.ce not taken up by the heart.

They are not simple sacs, however, but are separated into numerous divisions, as follows:

1. The lung on the right side of the thorax, called the right lung, is made up of three divisions, or _lobes_, and the left lung is made up of two lobes.

2. The lobes on either side are separated into smaller divisions, called _lobules_ (Fig. 33). Each lobule receives a distinct division of an air tube and has in itself the structure of a miniature lung.

[Fig. 33]

Fig. 33-*Lungs and air pa.s.sages* seen from the front. The right lung shows the lobes and their divisions, the lobules. The tissue of the left lung has been dissected away to show the air tubes.

3. In the lobule the air tube divides into a number of smaller tubes, each ending in a thin-walled sac, called an _infundibulum_. The interior of the infundibulum is separated into many small s.p.a.ces, known as the _alveoli_, or air cells.

The lungs are remarkable for their lightness and delicacy of structure.(30) They consist chiefly of the tissues that form their sacs, air tubes, and blood vessels; the membranes that line their inner and outer surfaces; and the connective tissue that binds these parts together.

All these tissues are more or less elastic. The relation of the different parts of the lungs to each other and to the outside atmosphere will be seen through a study of the

*Air Pa.s.sages.*-The air pa.s.sages consist of a system of tubes which form a continuous pa.s.sageway between the outside atmosphere and the different divisions of the lungs. The air pa.s.ses through them as it enters and leaves the lungs, a fact which accounts for the name.

[Fig. 34]

Fig. 34-*Model of section through the head*, showing upper air pa.s.sages and other parts. 1. Left nostril. 2. Pharynx. 3. Tongue and cavity of mouth. 4. Larynx. 5. Trachea. 6. Esophagus.

The incoming air first enters the _nostrils_. These consist of two narrow pa.s.sages lying side by side in the nose, and connecting with the pharynx behind. The lining of the nostrils, called _mucous membrane_ is quite thick, and has its surface much extended by reason of being spread over some thin, scroll-shaped bones that project into the pa.s.sage. This membrane is well supplied with blood vessels and secretes a considerable quant.i.ty of liquid. Because of the nature and arrangement of the membrane, the nostrils are able to _warm_ and _moisten_ the incoming air, and to _free it from dust particles_, preparing it, in this way, for entrance into the lungs (Fig. 34).

The nostrils are separated from the mouth by a thin layer of bone, and back of both the mouth and the nostrils is the pharynx. The _pharynx_ and the _mouth_ serve as parts of the food ca.n.a.l, as well as air pa.s.sages, and are described in connection with the organs of digestion (Chapter X). Air entering the pharynx, either by the nostrils or by the mouth, pa.s.ses through it into the _larynx_. The larynx, being the special organ for the production of the voice, is described later (Chapter XXI). The entrance into the larynx is guarded by a movable lid of cartilage, called the _epiglottis_, which prevents food particles and liquids, on being swallowed, from pa.s.sing into the lower air tubes. The relations of the nostrils, mouth, pharynx, and larynx are shown in Fig. 34.

From the larynx the air enters the _trachea_, or windpipe. This is a straight and nearly round tube, slightly less than an inch in diameter and about four and one half inches in length. Its walls contain from sixteen to twenty C-shaped, cartilaginous rings, one above the other and encircling the tube. These incomplete rings, with their openings directed backward, are held in place by thin layers of connective and muscular tissue. At the lower end the trachea divides into two branches, called the bronchi, each of which closely resembles it in structure. Each _bronchus_ separates into a number of smaller divisions, called the _bronchial tubes_, and these in turn divide into still smaller branches, known as the _lesser bronchial tubes_ (Fig. 33). The lesser bronchial tubes, and the branches into which they separate, are the smallest of the air tubes. One of these joins, or expands into, each of the minute lung sacs, or infundibula. Mucous membrane lines all of the air pa.s.sages.

*General Condition of the Air Pa.s.sages.*-One necessary condition for the movement of the air into and from the lungs is an un.o.bstructed pa.s.sageway.(31) The air pa.s.sages must be kept open and free from obstructions. They are _kept open_ by special contrivances found in their walls, which, by supplying a degree of stiffness, cause the tubes to keep their form. In the trachea, bronchi, and larger bronchial tubes, the stiffness is supplied by rings of cartilage, while in the smaller tubes this is replaced by connective and muscular tissue. The walls of the larynx contain strips and plates of cartilage; while the nostrils and the pharynx are kept open by their bony surroundings.

[Fig. 35]

Fig. 35-*Ciliated epithelial cells.* _A._ Two cells highly magnified. _c._ Cilia, _n._ Nucleus. _B._ Diagram of a small air tube showing the lining of cilia.

The air pa.s.sages are _kept clean_ by cells especially adapted to this purpose, known as the _ciliated epithelial cells_. These are slender, wedge-shaped cells which have projecting from a free end many small, hair-like bodies, called _cilia_ (Fig. 35). They line the mucous membrane in most of the air pa.s.sages, and are so placed that the cilia project into the tubes. Here they keep up an inward and outward wave-like movement, which is quicker and has greater force in the _outward_ direction. By this means the cilia are able to move small pieces of foreign matter, such as dust particles and bits of partly dried mucus, called phlegm, to places where they can be easily expelled from the lungs.(32)

[Fig. 36]

Fig. 36-*Terminal air sacs.* The two large sacs are infundibula; the small divisions are alveoli. (Enlarged.)

*The Alveoli.*-The alveoli, or air cells, are the small divisions of the infundibula (Fig. 36). They are each about one one-hundredth of an inch (1/4 mm.) in diameter, being formed by the infolding of the infundibular wall. This wall, which has for its framework a thin layer of elastic connective tissue, supports a dense network of capillaries (Fig. 37), and is lined by a single layer of cells placed edge to edge. By this arrangement the air within the alveoli is brought very near a large surface of blood, and the exchange of gases between the air and the blood is made possible. It is at the alveoli that the oxygen pa.s.ses from the air into the blood, and the carbon dioxide pa.s.ses from the blood into the air.

At no place in the lungs, however, do the air and the blood come in direct contact. Their exchanges must in all cases take place through the capillary walls and the layer of cells lining the alveoli.

[Fig. 37]

Fig. 37-*Inner lung surface (magnified)*, the blood vessels injected with coloring matter. The small pits are alveoli, and the vessels in their walls are chiefly capillaries.