Before leaving the dissection we have been surveying it will be well to see one of those marvellously contrived structures known as a joint. The wrist joint is still hidden by the tendons; even when these are cut through the interior of the joint is not yet visible; it is enclosed by stout bands of tissue or ligaments which become tight when the joint is over-bent. They prevent dislocation of the joint; indeed, so strong are those of the wrist joint that when we stumble forwards, or fall on the outstretched hand, it is the bones and not the ligaments which are apt to give way. When the ligaments are cut through, the articulating or jointed surfaces of the bones are seen. They are covered by an exceedingly smooth coating of white cartilage. Here, again, there is a self-lubricating mechanism which reduces friction at the joint to a minimum. In those individuals, however, who have the misfortune to suffer from rheumatism the self-lubricating mechanism has failed, the cartilaginous covering has become dry and worn away, and instead of a joint which works smoothly and silently there is one which is rough and creaks like a gate swinging on a rusty hinge.
We have surveyed the anatomy at the wrist in some detail and with a very distinct purpose. At every part of the limbs—upper and lower—we see the same arrangement of parts as at the wrist. There is first a covering of skin, then a layer of subcutaneous tissue, which unites the skin loosely to the third wrapping—the deep fascia. Within the sleeve of deep fascia are packed the muscles which move the limbs, the nerves which control the muscles and supply sensation to the parts; the great arteries which carry the nourishing blood from the left ventricle of the heart, and the great veins which return the used blood to the right ventricle—the pump of the lungs. When the fleshy or perishable parts are removed by dissection or by the corruption which so soon overtakes the soft parts after death, only the bones or skeleton remain to represent what was at one time a marvellous living machine.
We now propose to transfer our attention for a short time to two students who are uncovering the parts in front of the neck between the chin and breastbone or sternum. The windpipe has already been exposed, and is seen issuing from the voice-box or larynx below the chin to disappear at the upper opening of the chest on its way to the lungs. On each side of the windpipe the carotid arteries are found passing upwards to supply the head and brain with blood ; close by them are the jugular veins carrying the venous blood in an opposite direction. Here we have an opportunity given us of seeing a peculiar feature of man's structure. Just above the larynx the carotid artery divides into two branches, an external one which nourishes the face, and an internal one which supplies the brain with blood. Man has a large brain and a relatively small face, hence in him the internal branch is the larger. In all other animals the external is much the larger, because the face is massive while the brain is small. It has been suggested that our brains are large because of the calibre of our internal carotid arteries ; that statement we do not believe any more than the word of the waggoner who assures us that it is the dray which pulls the horse. Our object, however, in examining the anatomy of the neck is to see that curious structure or gland known as the thyroid body. It is made up of two parts or lobes, one on each side of the larynx and upper part of the windpipe; the lobes are united together by a part which crosses in front of the windpipe. Most glands in the body, such as the salivary and liver, have ducts or channels by which is discharged the substances they secrete, but there is no duct connected with the thyroid. The secretion which it forms is discharged directly into the blood stream and hence it is called a ductless gland or a gland of internal secretion. In recent years we have come to recognize that the secretion of the thyroid body is of the greatest importance. In children who suffer from disease of this gland we see that the growth of their bones is delayed or ceases, their skin becomes pasty, puffy and ill-nourished, and what is more serious their brains do not develop properly, and they become cretins or idiots. In some parts of this country—especially in Derbyshire— the thyroid is apt to become enlarged, forming a goitre and giving rise to the condition popularly known as " Derbyshire neck." There are other ductless glands, such as the pituitary body which lies enclosed within the skull and below the brain, and the suprarenal bodies which are situated in the abdomen above the kidneys. Our sense of well-being, our capacity for work and for pleasure, the nourishment and growth of our bony frames depend to a very great extent on the manner in which these small, insignificant-looking ductless glands perform their proper functions.
Our time with the students in the dissecting room has almost expired ; there remains only a moment to glance at a dissection which is exposing the important organs which are enclosed within the thorax and abdomen. Part of the front wall of these cavities has been removed. Within the thorax we see the heart enclosed within its fibrous sac— the pericardium. Two great arteries issue from its upper part—the pulmonary artery to convey the impure blood from the right ventricle to the lungs, and the aorta from the left ventricle to nourish the body with pure blood. Two great veins enter the right side of the heart—the upper and lower venae cavae; they bring back the impure blood gathered from the various parts of the body. The pulmonary veins convey the pure blood from the lungs to the left side of the heart. Within the thoracic cavity are the two lungs, one on each side of the heart. They are mottled and dark with soot, showing that their owner had breathed the air of these who live in large cities.
At the moment we have chosen to view the students at work two of them are examining that wonderful partition—the diaphragm —which separates the chamber containing the heart and lungs from the lower or abdominal cavity in which the organs concerned with digestion are placed. Thanks to the discovery of Rontgen these students have a decided advantage over their predecessors of fifteen years ago ; they can see the diaphragm, which is mainly composed of muscle, actually at work in your body or mine. As we take a breath the domes of the diaphragm are seen to descend, enlarging the cavity of the thorax, and we see the lungs become clearer as they expand and are filled with air. We can also see the dark shadow of the liver descending below the right dome of the diaphragm and the transparency that marks the stomach pushed downwards under the left dome. As we allow our breath to escape we see the domes of the diaphragm again ascend, and if we place our hand on our bodies as we breathe we shall observe that, as the diaphragm ascends, the muscles which enclose the abdomen are at work, pressing the viscera and the diaphragm upwards and thus returning the parts to a proper position for taking another breath. All the muscles which we now see connected with the walls of the cavities of the thorax and abdomen are concerned in respiration. At the moment of birth they begin to work and keep on unceasingly all through the years of life until death brings to a final stop one of the most wonderful mechanisms of the human body. We have not the time now to look at the nerves and nerve centres which control the muscles of respiration and keep them at work both when we sleep and when we wake.
There are structures connected with digestion which we might examine, but we must postpone their consideration until another opportunity. It may have occurred, however, to the onlooker that, since we can trans-illuminate the human body, it is no longer necessary to dissect it. Dissection is still necessary, for we cannot interpret correctly what is seen when the body is lighted up under X-rays unless we already possess an extremely accurate knowledge of the arrangement of parts as they are displayed in the human body after death.
Our cursory visit to the dissecting room has not been in vain if the reader has realized how complex the structure of the human body really is, and how necessary it is that those who have to cure its disorders should try to understand the intricacy of its mechanism. We have seen, however—and this is of more importance for our present purpose— the manner in which our knowledge of the human body is obtained. What one generation of anatomists has learned is written in books and thus handed on. For more than three centuries men have studied the structure of the human body, and yet to-day there is still much, very much, which we do not understand, but we live and work in the hope that our knowledge will continue to increase.