WE have talked of the mechanism by which air is drawn into the lungs by enlarging the diameters of the chest, and we must now describe the passages which convey the air down to the lungs. During normal breathing air is first drawn into the nose which has a special action in warming and moistening the air and clearing it of some of its larger impurities before it reaches the delicate tissues of the lungs. When the air enters at the nostrils, it finds itself in quite a large cavity whose depth is represented on the front of the face by the distance from the nostrils to the bridge of the nose.
If you can imagine a cavity as deep as this and perhaps one and a half inches from side to side, extending directly backwards until it joins with the cavity of the mouth, you will have quite a good idea of the anatomy of the nose. This cavity is divided into two by a partition which runs from the roof to the floor and completely separates one side from the other. At the entrance of each nostril there are a large number of very stiff hairs which strain off any particles such as soot which might be breathed in.
The side wall of each compartment of the nose is thrown into a number of large folds which greatly increase the surface which is presented to the air passing over it, and as they project inwards towards the partition they take up a lot of room and in places almost touch it. The whole of the interior of the nose, including these folds, is covered with a thick, velvety layer of mucous membrane which is very thickly supplied with blood vessels. It is called mucous membrane because it is a membrane made up of cells that secrete a thick viscid fluid known as mucus. As the mucous membrane is well supplied with blood it is always kept warm and the mucus secreted keeps it soft so that the air passing over it has the chill taken off and is moistened.
In addition some of the dirt carried in with the air adheres to the mucus and is discharged from the nose when next it is blown. That this is so is well shown by the difference in the dirt on our handkerchiefs after a day spent in the smoky atmosphere of a large city and that found after a day spent in the country. It must be realised that all this filth would have entered the lungs, where it might have done
considerable damage, if it had not first been removed by the nose.
The nose is, of course, one of the main places where germs enter the body, and considerable numbers are caught and held up in the mucus which covers its surfaces. Very frequently these are germs which cause trouble and a cold in the head is the result. When this happens the mucous membrane of the nose becomes swollen, so much so that the nose is blocked and breathing through the nose becomes difficult or impossible. At the same time the flow of mucus is greatly increased so that large numbers of the disease-producing germs are washed away in it.
THE SECRET OF A NASAL VOICE
WE have seen that the nose communicates at the back with the cavity of the mouth. The posterior part of the floor of the nose is formed by a muscular sheet known as the soft palate. When this is pulled up, it shuts off the back of the nose from the mouth so that when we swallow fluids they cannot enter the nose and flow out through the nostrils.
Leading out of the cavity of the nose are quite a number of fairly large cavities, whose function it is to make the sounds formed by the larynx reverberate, so that the voice takes on a ringing character. When we speak the nose and mouth are kept in continuity because the soft palate is kept open and air passes through the nose as well as through the mouth. If for any reason the nasal cavity is blocked, as for example when we have a cold in the head or when we pinch the nostrils together with our fingers, the voice will be flat and ‘nasal ‘in quality. The action of these cavities is exactly like that of the large cavity which is always made in the body of a violin behind the strings. One can readily imagine the flatness of the note of a violin that was made of a solid block of wood.
THE NERVES THAT ENABLE US TO SMELL
THE roof of the nasal cavities is formed by quite a thin plate of bone which separates the nose from the cavity in which the brain lies. Piercing this layer of bone are numerous tiny nerves which carry sensations of smell to the brain. Being extremely delicate structures Nature has packed them away in an inaccessible region of the nose where the least possible harm can come to them, and yet where the air warmed
by the remainder of the nose and carrying the odorous substances which stimulate these nerves can easily reach them.
Owing to the close proximity of the brain to the outside wall at this point it is one of the situations in which the brain is most exposed to infection, and it is one among others at which it is supposed the germs of infantile paralysis enter the nervous system. This disease is an example of many such that are caught first in the nose, just like the common cold.
THE SOUND BOX OF THE HUMAN VOICE
BOTH the nose and the mouth lead backwards into a cavity which connects the two and which is known as the pharynx. This has two large tubes leading from its lower portion. The front one is the larynx, which produces the voice, and the one lying behind is the gullet. Leading out of the side wall of the pharynx on each side is a short tube that connects it with the inside of the ear. This is one of the danger spots of the upper respiratory passages, for quite often when these are inflamed owing to some infection, such as measles, this tube also becomes inflamed and infection may reach the cavity of the ear. This is the explanation of the relative frequency of that serious condition known as mastoid disease which so often follows otherwise trivial diseases of this region.
Leading out of the lower part of the pharynx is the cavity known as the larynx. This is like a box with its upper and lower ends open and is formed by a number of small cartilages, the largest of which projects on the front of the neck and is known as the Adam’s apple.
Stretching from front to back of the lower part of this cavity are two folds of mucous membrane known as the vocal cords. When air passes over these structures during expiration and inspiration they are thrown into vibration and a sound is produced. They are well supplied with different sets of muscles so that not only can they be separated and brought closer together so that they may be made to meet completely, but also they can be both stretched and relaxed. Separation and approximation of the cords will control the volume of air passing over them so that the loudness of the voice can be controlled. Tightening and relaxation of the cords determines their pitch so that all kinds of variation in the tone of the voice can be produced. The larynx pro-
duces only the sound of the voice. It is the mouth and tongue, lips and teeth, that fashion these sounds into words and syllables.
WINDPIPE AND BRONCHI: THE SENTINELS OF THE LUNGS THE lower part of the larynx leads directly into the windpipe which runs down through the lower part of the neck into the chest. Here it divides into two branches and these in time divide into even smaller and smaller branches until eventually they reach the air sacs of the lungs.
The windpipe and bronchi are composed of rings of cartilage separated in the intervals by fibrous tissue. The rings are incomplete behind and the interval between the ends of the ring behind is filled in by muscular tissue, thus enabling the tube to be made smaller or larger in diameter when the muscle contracts or when it relaxes. This is of little importance in the windpipe, for it is a very wide tube and only slight variations can be made in its calibre. But in the smaller bronchi it is of some importance, for it is the explanation of how asthma is produced.
There are certain substances in the air we breathe which will cause a spasm of these muscles in individuals who are susceptible to them. Such things are the pollens of certain plants and dusts from various sources. When the substances gain access to the bronchi the bronchial muscles go into spasm in an effort, as it were, to exclude the dangerous particles from the lungs. So effective is the spasm that it often makes breathing almost impossible for the unfortunate sufferer.
It is really misdirected energy on the part of the bronchi, for the condition they produce is more unpleasant and more dangerous than the one they are seeking to avoid. It is therefore justifiable for the doctor to abolish this spasm by the use of drugs, and this is usually done by inhaling the smoke from certain leaves or by giving a special injection. When the bronchi become inflamed the well-known condition of bronchitis is produced.
The mucous membrane of the windpipe and bronchi is very remarkable and forms the final barrier which tends to prevent dangerous substances entering the lungs. It is composed of a layer of tall cells fitted together like a palisade. Each cell has a number of hairs sticking out into the cavity of the windpipe, which the cell can move freely. There are small glands lying deep in the membrane which throw out
quantities of fluid mucus in which foreign particles and germs become entangled.
The hairs on the cells beat always upwards so that they carry the mucus which is laden with foreign particles upwards in a stream away from the lungs. This is eventually voided by coughing, because when it reaches the sensitive larynx a reflex is set up which results in the production of a sharp cough. This same reflex is responsible for the unpleasant results that follow food ‘going the wrong way,’ for instead of being swallowed normally it is inhaled by mistake into the larynx from which it might reach the lungs if it were not immediately coughed up.
THE RHYTHMIC WORKING OF THE LUNGS
THE lungs consist of a vast number of infinitesimally small air sacs crowded together and welded into one mass so that a structure something like a very fine sponge is produced. Each tiny air sac has a correspondingly small branch of a bronchus leading into it, so that during inspiration and expiration the air lying in the air sacs is constantly being changed and kept fresh. The walls of the sacs are formed by a very thin membrane formed by flattened ends which lie only one layer deep, so that an effect like a pavement is produced.
Running in the walls of the sacs and separated from their air-containing cavities by only this thin layer of membrane are innumerable capillaries which are branches of the pulmonary artery that brings impure blood from the right side of the heart for purification. So fine and so numerous are the capillaries that an enormous surface of blood is exposed to the action of the air in the sacs, with the result that the gaseous interchange between blood and air can take place to the best advantage.
The substance of the lungs is made up mostly of elastic tissue, so that when they have expanded under the influence of the increased size of the chest during respiration, they will automatically collapse and drive out the air inside them, when the expanding force is removed. The outer surface of the lungs is covered with a smooth, glistening membrane known as the pleura. The whole of the inner surface of the chest is also lined with this same membrane, the result being that the lungs can move about inside the chest when they are expanding and relaxing with the minimum amount of friction.