the vertebra, from the Latin verto (I turn). Each is further connected with the other by certain grisly elastic substances, which are called the intervertebral cartilages. Now of these twenty-four vertebræ of which the spinal column is composed, seven belong to the cervix or neck, and are termed the cervical vertebræ; twelve constitute the dorsum or back, and are called the dorsal vertebræ ; and the remaining five have received the name of the lumbar vertebræ, from lumbus, the loin. You will perceive that the dorsal vertebræ have each four articulating processes, as they are termed, two transverse ones and a spinal one. The two transverse processes stand out on each side, and serve as places for the attachment of the ribs.

Let us now pass on to the consideration of the ribs. In general, I must tell you, we are furnished by nature with twenty-four, twelve on each side; but occasionally we meet with cases in which this normal number of twelve ribs on each side is either increased or diminished by one or two ribs. You perceive that the ribs are articulated behind with the dorsal vertebræ, and in front with the sternum or breast-bone.

I suppose I need hardly tell you that the belief which some of the uneducated classes still seem to entertain, that man has one rib less than woman, arising no doubt from the narrative given in the Book of Genesis regarding the formation of Eve, is simply a vulgar and absurd error. The upper seven ribs, to which I am now pointing, are called the true or sternal ribs, because they are immediately connected with the sternum, or backbone, by means of cartilages. In contradistinction to these upper seven ribs, the lower five are called the false ribs. The last two of these ribs are floating, but otherwise they are supported by the breast-bone, and

cartilaginous appendages attach the two floating ribs to each other and to the one above.

The breast-bone in early life consists of various pieces, of which two can be distinctly seen even in manhood. You will see that its lower extremity has an appendage bearing some resemblance to the end of a sword. From this it has been termed the ensiform cartilage, from ensis, a sword. It is not till quite an advanced period of life that this finally ossifies. You will also notice that the breast-bone has on either side seven depressions. These are for the purpose of giving admission to the cartilaginous extremities of the upper seven or true ribs. They are not articulated with the spine at right angles, but take a slanting direction downwards, an important modification, because upon this chiefly depends what is termed costal respiration.

So much then for the osseous or bony portion represented in the diagram. I must now speak of the muscles which carry on the function of respiration. I may say, generally, that most of the muscles connected with the trunk are indirectly concerned in aiding the function of respiration; but the direct muscles which regulate the respiratory actions are the following, viz., the intercostals, the elevators of the ribs, the triangular muscle of the breast-bone, and the serrated muscles on the back. All these are directly concerned in elevating the ribs so as to enlarge the capacity of the chest. But I must tell you that the principal agent in carrying on ordinary respiration is this to which I now point, forming, as you see, a partition between the contents of the chest and those of the abdominal regions. It is hence called the diaphragm, and may be said to form the floor of the chest, and the roof of the abdominal cavity. To the former it is convex in shape, and to the latter concave. Though you often hear the diaphragm spoken of as a single muscle, it really consists of two muscles and a central tendon. It is also worthy of notice that the diaphragm takes a slanting direction from the breastbone to the loins. When in a state of relaxation the lateral borders, which are movable, present convex arches, which reach up sometimes as high as the fourth rib. On the other hand, the arches, when in a state of contraction, present nearly plain surfaces, by which the capacity of the chest is increased to the same extent as it was previously diminished by the diaphragm being relaxed.

It is right, however, that I should mention that a modern German physiologist of considerable reputation, Herr Merkel, has expressed his doubts whether the English physiologists have not attached too much importance to the diaphragm as an organ of respiration. He considers it to be not a direct but an auxiliary muscle in involuntary respiration, though he admits it becomes active in voluntary respiration, and when in consequence of disease the other respiratory muscles cannot easily act.

Now, then, let us examine a little in detail what is for our purpose the most important portion of the contents of the chest, or box, as the word literally means. In this large diagram which you see before you, you have a representation of the human lungs. (Fig. 2.)

You perceive that they consist of two bodies somewhat conical in shape. They are situated within the lateral cavities of the chest, and

are divided from each other by two layers of the pleura which form a membraneous partition called the mediasterium. Thus the lungs have no direct communication with each other. Between them is situated the heart. In front they are covered by the ribs, behind by the spine, and their bases rest upon that muscular floor I have just spoken of, the diaphragm. The weight of both lungs varies from thirty to forty-eight ounces, and not only are they absolutely heavier in man than in woman, but also relatively in proportion to the weight of the body: still they are the lightest viscera in the whole body. Each lung is divided into

parts called lobes, and these again are composed of smaller lobes, or lobules, as they are called. But the right lung is rather larger than the left, and has three distinct lobes, while the left has only two, and presents in its anterior border a deep notch, into which the apex of the heart, enclosed in the pericardium, is inserted. It is not the least exaggeration to say that the spongy substance of the lungs consists of millions of microscopic air-cells, varying in the adult from too of an inch in diameter. Indeed, by the eminent physiologist Kiel they were estimated to be about a hundred and eighty millions! I know scarcely any more

interesting object under a powerful compound microscope than a section of the human lung. It is through the trachea or windpipe that the air is conveyed into the lungs by means of the bronchial tubes, and thence it is carried by means of still smaller vessels into ramifications that at last are of the most minute size. I shall speak more fully of the mechanism of respiration subsequently, but I may just say at present that the philosophy of respiration may be thus briefly explained.

The air, so long as we have life, is of course continually passing in and out of our lungs, thereby oxygenating the blood, and then passing out again. The impure venous blood and the chyle produced by the digestion of our food are distributed over these millions of microscopic air-cells I have just spoken of by means of most minute vessels, called, from their extreme fineness, the capillary vessels, from the Latin capilla, a hair, and are derived from the pulmonary artery. They form a perfect net-work over the surface of each of these air-cells. It is during the circulation of the venous blood upon these air-cells that its properties and colour, too, are completely altered. For whenever we take in the air in the act of inspiration as it is termed (and hence the great importance of good, deep, steady inspirations as regards health, especially in a fresh, pure atmosphere), the blood and the air we breathe in are divided only by a membrane so marvellously fine and delicate, that while it is sufficiently thick to retain the blood, it yet allows the oxygen of the air and the impure gases of the blood freely, as it were, to filter through it; and this, indeed, is the special vital property of the membrane in question. A portion of the oxygen is received into the blood, changing its character from venous to arterial, which transformation is characterised by the colour of the blood passing from a dark purple hue to a bright red. The remainder of the oxygen then combines with carbonaceous compounds of the blood to form carbonic acid gas, which poisonous product is cast out in the act of expiration. And is it not a marvellous thing to reflect upon, that it is in this very act of casting out what is not only useless to the system, but a deadly poison if retained, that we are endowed with the wonderful power of communicating our thoughts to our fellow creatures in articulate speech? It has always appeared to me a most striking instance of the Divine economy, as worthy of admiration as the great problem which has been solved in the structure of the lungs, viz., to expose the largest quantity of blood to the contact of the air within the comparatively moderate space to be occupied by the lungs.

It may not be uninteresting for me to mention some curious facts arrived at by Dr. Hutchinson in regard to the capacity of the human lungs for containing air. After extensive experiments made with the instrument called the Spirometer, he found that a male adult of an average height can, after a complete inspiration, expel from the lungs. by a forced expiration, no less than 225 cubic inches of air, at a temperature of 60 degrees. Even after this forced expiration 109 cubic inches of air are still retained in the lungs, so that these two amounts being added together will give 335 cubic inches of air as the total capacity of the organs of respiration for air in the adult male of average

stature. Dr. Hutchinson also found that the capacity of the lungs bears a uniform relation to the height of the individual, increasing eight cubic inches for every inch above five feet; and, lastly, he discovered that when we are sitting or lying down there is a considerable diminution in the capacity of the lungs for containing air; being in the former case lessened from 260 to 255, and in the latter to 230 cubic inches. After a hearty meal, too, the capacity of these organs is lessened from ten to twenty cubic inches.*

And now, before I close this lecture, let me say a few words in reference to the trachea, or windpipe, and the bronchial tubes. You see that it is nearly a cylindrical tube, and forms the common air passage to both the lungs. It is partly situated in the neck and partly in the chest, and measures from about four to four and a half inches in length. Its structure consists of from sixteen to twenty cartilagenous rings, connected with each other by a ligamentous substance mixed with muscular fibres. It is to be noticed that these cartilagenous rings are, however, not quite complete, for they terminate behind in a muscular and ligamentous membrane, which, whilst completing the tube, admits yet of compression. Loose cellular tissue surrounds the trachea, so that it can move freely on the surrounding parts. You observe this other tube behind it. This is called the asophagus, or gullet. It is the canal down which all that we eat and drink passes. It is covered with a soft membrane, secreting a mucous fluid, which defends the surface from the acrimony of the air.

Now let us see the course which the trachea takes. You notice that after it has passed down the neck it divides as it enters the chest into two parts, one for each lung; but you perceive they differ from each other in size and direction. These two smaller tubes are called the bronchi (from the Greek word Bgoyxos, the throat), and you will remark that they differ from each other in size, and also in the course they take. The right bronchus is shorter but wider than the left, and is usually about an inch in length, while the left bronchus is nearly twice as long. Their general structure resembles that of the trachea. The number of rings in the right bronchus varies from six to eight; in the left from nine to twelve. Before penetrating the substance of the lungs, the bronchus divides again into further branches, one being intended for each lobe of the lungs. As they proceed, they still further ramify through the lungs, becoming smaller and smaller, and the cartilagenous rings less and less distinct, until finally they quite disappear, so that the air cells in which they terminate appear to be but an extension of the mucous membrane which lines the bronchi. It is evident that the cartilagenous structure of the trachea and bronchi serves to keep the air passages open; and it has been suggested that if we do not find them in the minuter branches, the probable cause is that they can never be completely emptied of air after the first inspiration of "the breath of life" has been taken at the time of birth.

Now, as the proper regulation of the act of respiration is one of the most important things to be attended to by the public speaker or reader as "Medico. Chi. Transactions," vol. xxix.