In the upper extremity of the thigh-bone there is a more complex system. The neck of the femur serves as a bracket to fix the head of the thigh-bone to its shaft. When we stand on one leg the whole weight of the body is transmitted by the neck of the thighbone. When the neck is laid open it is seen that the osteoblasts have laid the bone down, and are maintaining it in two main systems. One of these—the supporting system—passes down from the head of the thigh-bone to join the system of the shaft; the other system forms a tie series passing outwards transversely from the head to the upper end of the shaft. The bone systems thus described in the neck of the thigh-bone correspond to the oblique and horizontal rods seen in the brackets of old-fashioned street lamps. If, however, a fracture of the femur occurs and the lines of force or of support are altered then the osteoblast will meet the new conditions by rearranging the system of bone trabeculae or needles.
It is only in short bones or at the extremities of long bones that the osteoblasts lay their material down in specially arranged needles, plates or spongy patterns. In the shafts of long bones the material forms a hollow cylinder with dense walls. Engineers are well aware that the strongest support is obtained with the least expenditure of material by using hollow cylindrical pillars.
There is no end to the wonderful mechanisms of the human body ; many chapters would be required to deal with them exhaustively. Some of the most beautiful examples are seen in connexion with the heart and lungs, but we will put these aside and merely mention one which excited the admiration of Archdeacon Paley. " It has been said," he writes, " that whenever nature attempts to work two or more purposes by one instrument she does both or all imperfectly. Is this true of the tongue regarded as an instrument of speech, and taste and deglutition ? So much otherwise that nine hundred and ninety-nine persons out of a thousand, by the instrumentality of their one organ, talk, taste and swallow very well. . . . There are brought together within the cavity of the mouth more distinct uses of parts executing more distinct offices than I think can be found lying so near to one another, or within the same compass in any other portion of the body." That is true, and during the century which has elapsed since his words were written we have become aware that the mechanism of mastication, of speech and of swallowing is more complicated than even Archdeacon Paley supposed. During mastication the mandible undergoes an extremely complex series of movements, which are regulated and controlled at each phase by a special system of muscles and nerves. The carpet of glands spread everywhere beneath the mucous membrane of the mouth, lips and tongue, is reflexly called into action by another nerve mechanism and throws out a lubricating secretion which moistens the lips, cheeks and gums. The three pairs of salivary glands situated in the cheek (parotid) and under the jaw (submaxillary), and under the tongue (sublingual) are stimulated by another system of nerves to pour their secretions into the mouth. At meal times neither speech nor respiration need be interrupted.
The mechanism of the throat or pharynx is also complex. A bolus of food, when it has been passed from the mouth to the pharynx in the first stage of swallowing, finds four passages open for its further progress. It may return to the mouth, it may pass into the naso-pharynx and nose : it may pass into the larynx or windpipe. These three entrances or passages to the pharynx are kept open except during the act of swallowing. There is a fourth passage or opening leading to the oesophagus which is always closed except when pushed open by a bolus of food or mouthful of fluid. Before the bolus of food which has just reached the pharynx can be seized by the muscular wall of that cavity and propelled onwards, it is necessary that the passages to the mouth, nose, and larynx be shut and the opening to the oesophagus relaxed or open. An elaborate system of muscles, under the control of an automatic nerve mechanism, accomplishes all these things with every mouthful we swallow. Swallowing seems such an easy and automatic act that we are quite unaware of the elaborate system of signals, side shunts and level-crossings which have to be manipulated to permit the busy traffic of the pharynx to pass unchecked. A bolus of food in the pharynx brings reflexly all of these into action, but there are occasions when their efficiency is disturbed by an urgent and unexpected message from the larynx or the nose. In other words, a morsel of food in the windpipe which has passed the sentry at the larynx at an absent minute calls other muscles into play and upsets the normal mechanism of swallowing. The movements carried out under the active guidance of the brain form only a fraction of the work which goes forward in the body. The elaborate movements of respiration, the mechanism of the heart; of the arteries, of the veins, the regulated contractions of the stomach and bowel, besides many movements of the body, are being conducted and regulated day and night by nerve centres and systems over which we have little or no control.