The movements of the limbs.—An excellent illustration of this relation between the bones and muscles is seen in the bending of the arm at the elbow. When the arm is bent, a great mass of flesh, the biceps muscle, in front of the upper arm may be observed to become much thicker. The muscle ([Fig. 160]) thickens because it becomes shorter. Its upper end is attached to the corner of the shoulder-blade (a), which remains stationary; while its lower end is connected at P with one of the bones of the fore-arm. The shortening of the muscle therefore draws up the fore-arm. The elbow-joint, about which the motion takes place, is a very perfect hinge. Several other forms of joint are also found in various parts of the body. It is seen, when the skeleton is examined, that in every case the characters of the joints and the attachment of the muscles are most admirably adapted to the movements with which they are associated.
The study of the skeleton.—The study of the rabbit’s skeleton is not only highly interesting in itself, but necessary for the intelligent appreciation of the animal’s life. And when it is compared with the skeletons of other familiar animals, a common plan of structure is found which illustrates in the most convincing manner the kinship which often exists between very dissimilar creatures. Such a comparison shows that, almost bone for bone, the skeleton of a rabbit corresponds with that of a man or a horse, and even with that of a bird or a frog. Mounted skeletons are shown in most natural-history museums, and the student should, whenever possible, examine and compare them.[9] He can himself, however, easily separate the bones from a boiled rabbit, and make out their main features and relationships.
The rabbit’s skull and backbone—The bones of the head are collectively known as the skull. This consists of (1) a large brain case; (2) the cavities for the organs of special sense, viz., (a) a pair of nasal chambers (in which the organ of smell is located) in front: their hinder ends open into the top of the throat; (b) the eye-sockets at the sides, and (c) the flask-shaped chambers for the internal ears at the sides of the hinder end of the brain-case; (3) the jaws: the upper jaw is rigidly attached to the brain-case, but the lower jaw is hinged at each side on the hinder end of the bony arch which runs below the eye. Both upper and lower jaws bear teeth, which are fixed in sockets. On each side the upper jaw contains two incisor teeth ([p. 219]) and—much further back—six grinding teeth. In the lower jaw are one incisor and five grinding teeth on each side.
Fig. 161.—The Rabbit.
The skeleton in position, seen from the left side.
The vertebral column or backbone is a chain of bony rings or vertebrae which runs dorsally ([p. 217]) from the hinder end of the skull to the tail, and forms a long tube containing the spinal cord—a backward continuation of the brain. On the anterior face of the first vertebra are two hollows into which a pair of knobs on the skull fit. Although, with the exception of the first and second, all the vertebrae are formed on essentially similar lines, there is considerable variation in shape and size in the different regions of the spine, as may be seen from [Fig. 161]. There are seven neck vertebrae, and this number is remarkably constant in mammals ([p. 220]). Following these are the chest vertebrae, which bear pairs of ribs. The majority of the ribs curve round and join on to a ventral bar of bone called the sternum or breast bone; so that the cavity of the chest, containing those very important organs the heart and lungs, is enclosed in a protective bony cage. The vertebrae of the abdominal region of the body are very large and stout. Between them and the tail-bones are four fused vertebrae, forming a mass which on each side gives attachment to the large hip-bone.
The bones of the rabbit’s limbs.—The fore and hind-limbs of the rabbit are obviously comparable: the upper arm corresponding to the upper leg, the elbow to the knee, the fore-arm to the “leg,” the wrist to the ankle, and the fore-foot to the hind-foot. This similarity becomes even more apparent when the limb-skeletons are examined. Commencing in each limb at the end nearest the body we find a single long bone. In the fore-limb the rounded, upper end of this bone works in a socket at the anterior angle of the shoulder-blade, a triangular plate on each side which overlies the chest dorsally; in the hind-limb the rounded end of the corresponding bone works in a cup in the hip-bone. Again, between the elbow and the wrist are two bones lying side by side; and between the knee and the ankle are two corresponding bones, although here they are only separate in their upper parts. Similarly, the wrist bones correspond to the ankle bones, and the bones of what may be called the fingers to those of the toes. Certain of the ankle bones are, however, much elongated: obviously a great advantage to an animal which progresses by a series of hops—owing to the increased leverage which is thereby given to the hind-foot. The rabbit’s fore-foot bears five toes, the hind-foot four.
The structure of a long bone.—The long bones of the limbs are hollow except at the ends. Strength and lightness are thus secured by a device (the hollow cylinder), which has already ([p. 72]) been seen to be adopted by the supporting structures of plants, and is also copied by human engineers. The cavity of the tube is filled by marrow, which supplies the bone with food. The hard, bony tube itself is partly composed of mineral matter and partly of animal matter. The mineral matter is left as a white, brittle framework when the animal matter is burnt away; while on the other hand the mineral part—which gives the bone its rigidity—may be dissolved out by immersing the bone in dilute hydrochloric acid, leaving the organic tissue as a soft flexible substance having the shape of the original bone.