Lower Extremities. At about the middle of the innominata are two cavities, 2. 4. into which are inserted the thigh-bone or femur, forming what is called a ball and socket joint. The extremity of this bone articulates with the tibia, the large bone of the leg, the fibula, or smaller bone, being firmly bound to it at the knee, forming a hinge-joint. Over this joint, affording it protection, is placed a smaller bone connected with the femur and leg bones by ligaments and muscles. It is called the patella or knee-pan. At the lower extremity of the leg we have the small tarsal bones, seven in number, forming the ankle. Articulating with one range of these bones are the metatarsal, five in number, to the extremities of these are connected the bones of the toes, called the phalanges of the toes.
Fig. 7. represents the bones of the ankle and foot. The tarsal or ankle-bones, 1, the metatarsal, 2, the phalanges of the toes, 3, 4, 5.
The Upper Extremities. The clavicle (collar-bone) is attached at one extremity to the sternum or breast-bone, at the other it is united to the scapula (or shoulder-blade.) It keeps the arm from sliding forward. The shoulder-blade is situated on the. upper and back part of the chest and is held in its position by muscles. The humerus is united by a joint with the scapula, and at the elbow it is articulated with the ulna of the fore-arm. This bone is on the inside of the arm, while the radius, which articulate with the carpus, forming the wrist-joint, is on the outside. These bones at their extremities articulate with each other, the upper end of the radius, rolling on the ulna, and the lower end of the ulna roll-ing on the radius, thus permitting the varied and beautiful movements of the arm. The carpus, or wrist, is composed of eight small bones. Articulating with one range of these, is the metacarpus, composed of five bones, forming the body of the hand. United to these are the bones of the fingers, called the phalanges of the fingers. The articulation of the bones are covered with cartilage, a substance of the nature of bone, yet smooth, solid and much softer. Covering the cartilage and forming around the joint a shut sack, is the synovial membrane. It secretes a serous fluid, which serves to lubricate the joint so that the motion may be free, easy, and without pain. The joints are kept in their position by ligaments or shining, strong, and elastic bands, which generally surround the whole joint.
The bones are composed of animal and earthy matter, the earthy part, giving them strength and solidity, while the animal imparts vitality. In infancy the animal substance preponderates, causing the bones to be softer and and more liable to bend than in old age, when the earthy preponderates, leaving the joints stiff, the bones brittle and liable to break. Over the bones is spread a thin membrane called the periosteum. This membrane may become inflamed, when it is peculiarly sensitive and painful.
We have thus the frame-work of the human system. The bones are all in their places, but they are only inanimate bones, without life, unable to perform a single movement. Let us then take another step, cover them with muscles, and thus attach to them the bands and pulleys by means of which movement is to be performed.
The Muscles. On this part of the subject we shall of necessity be short, as a minute description would lead us into those dry and technical details, which would he uninteresting to the general reader.
The muscles are composed of bundles of small fibres enclosed in a membranous investment or sheath. Towards the end of the muscles the fibres gradually change into the tendons or cords, by which they are strongly attached to the bones.
The muscles not only enable us to move, perform respiration and the various duties of nature essential to life, but give form and beauty to the frame, form those cavities, within which are enclosed important organs requiring their protecting covering.
The prominent characteristic of the muscles is, contractility on the application of the necessary stimuli, and relaxing when that stimuli is withdrawn. The natural stimuli is the will, which, flashing like lightning along the nerves, causes the muscles to relax, or contract, and produces those varied and rapid movements of which the body is capable. Each movement or expression is the result of the contraction and relaxing of the appropriate muscles. It is in this way the eyes are opened or closed, the mouth extended, contracted, opened, or closed, the face wears an expression of grief, or is merry with smiles, or convulsed with laughter. The jaw performs its rapid movements in mastication in this manner; breathing is performed by the contraction and ex-pansion of the muscles of the chest and abdomen. The rapid movements of the fingers in the musical performer and the varied and rapid motions of the dancer, show how obedient the muscles are to the mandate of the will. In the upper and lower extremities the muscles which produce flexion and those which produce extension are placed on opposite sides. In the lower limbs the flexors are placed on the posterior and the extensors on the anterior sides, while in the upper limbs their situation is directly the opposite. The strength and health of the muscles is increased by proper exercise. This fact is familiar to all, although all do not act upon the common-sense lesson it teaches, and take the necessary exercise.
Thus we perceive when the stimuli is applied to one set of muscles, they contracting, flex or draw up the limb. Shut off the nervous stimuli from this class and direct it to another, and their fibres contract, and the limb is extended. Thus, by this beautiful process the chest expands, the bosom rises and falls, we eat, drink, move and live. If the world would more frequently follow the plain and simple teachings of nature, there would be far less sighing and groaning than at present. See Plate 3.
We have now clothed the before naked and unsightly skeleton with flesh, giving it beauty and symmetry of form. Still there is no movement; all is as still and quiet as the form of the dead. The machine is ready for action, but has not yet felt the quickening influence of life. Let us then advance another step and examine the seat of that power, whose quickening influence is infused throughout the system, and whose mandate the muscles hasten to obey.