The thigh bone, “femur,” is the largest single bone in the body; it is joined to the pelvis by a ball and socket joint, an arrangement which permits its motion in any direction. The femur is supported by the bones of the leg. These are two in number in each leg, the tibia and fibula. The former is much the larger, and the latter is often called the “splint bone.”
The bones of the arm corresponding to those of the leg are the humerus, the radius, and the ulna. The humerus is the bone of the upper arm, and is large and strong; it articulates with the body in so perfect a manner that the great variety of motions required to be performed is easily and gracefully accomplished. This bone is fastened by a ball and socket joint, but is not, like the thigh bone, firmly fixed to an immovable bone. It is attached to a broad bone called the scapula, or blade bone; this is fastened to the body by muscles which give it great freedom of motion. The bones of the fore arm are the radius and ulna: these are nearly of the same size; in this they differ from the leg bones.
The wrist, composed of eight bones, forms the support for the tendons, blood vessels, and nerves which pass to the hand. To these are attached the metacarpal bones which support the phalanges, or bones of the fingers. The arrangement of the bones of the foot is, in many respects, like that of the hand. However, the bones of the foot form an arch which acts like a spring when the weight of the body falls on it, thus helping to ward off shocks and jars.
The skull is a large bony case in which the brain is lodged, and by which it is protected. It is composed of several pieces firmly united by a very irregular line of union. The attachments of the skull are the jaw bones, nasal bones, and the cheek bones.
Joints have been provided wherever parts of the skeleton require motion; they are of several kinds, chief among which are the ball and socket, hinge, and flat joints. The hip has been mentioned as a perfect example of the ball and socket joint, while the knee presents an example of the hinge.
Muscles are necessary to all motion, and are found everywhere throughout the body. They are the active agents, which, under the control of the nerve centers, do all the work necessary to carrying on the functions of the organs. They are endowed with a remarkable property called contraction. How this is accomplished is not fully understood, but when stimulated by nerve force or electricity they shorten and widen. This brings their ends nearer together; also the parts to which they are attached. They are always attached to movable parts; their attachment is a tendon at one end, and a fan-like distribution of their fibers at the other. Tendons are inelastic fibrous tissue, very strong, and consequently much smaller than the muscle. They are to muscles what tugs are to horses. Muscles are either flexors or extensors; the former bend the joints, the latter straighten them.
The skin forms an exterior covering for the body, and is employed for other and very important functions. Its appendages are the hair and nails. Some of the uses of the skin are these: It is highly sensitive and capable of distinguishing various sensations; it has the property of excreting, of secreting, and of absorbing. It is an organized texture containing blood vessels, glands and nerves. It is composed of several layers; the outside, called the epidermis, is not supplied with blood vessels, and is produced by the under layers.
The glands of the skin are first, those which secrete the perspiration. This is poured out to keep the temperature of the body even, also to rid the circulation of many injurious substances. Then there are glands which secrete an oily matter which aids in preserving the softness and pliability of the skin. These glands are called sebaceous glands. Hair is said to be an appendage of the skin because its texture is essentially similar to that of the cuticle. Nails, also, are but modifications of the skin. The fact that the epidermis is being constantly cast off and renewed, and that matter is both secreted and excreted continually, renders frequent washing of the skin necessary to health.
Digestion—In animals the arrangements which exist for converting aliment into blood are more or less complicated according to the requirements of each class; and this conversion is called assimilation, a term which includes digestion and those changes which take place in the blood and tissues, by which new material is added to them.
The preparation of the food for assimilation by the tissues is accomplished in a long tube called the alimentary canal. This canal is made up of various parts having different functions and different construction. These parts we will briefly describe. They are the mouth, pharynx, æsophagus, stomach, small and large intestine.