Experiment 6. Trim off the flesh from the second half. Note the pinkish white appearance of the bone, the marrow, and the tiny specks of blood, etc. Knead a small piece of the marrow in the palm; note the oily appearance. Convert some marrow into a liquid by heating. Contrast this fresh bone with an old dry one, as found in the fields. Fresh bones should be kept in a cool place, carefully wrapped in a damp cloth, while waiting for class use.
A fresh or living bone is covered with a delicate, tough, fibrous membrane, called the periosteum. It adheres very closely to the bone, and covers every part except at the joints and where it is protected with cartilage. The periosteum is richly supplied with blood-vessels, and plays a chief part in the growth, formation, and repair of bone. If a portion of the periosteum be detached by injury or disease, there is risk that a layer of the subjacent bone will lose its vitality and be cast off.[5]
30. Microscopic Structure of Bone. If a very thin slice of bone be cut from the compact tissue and examined under a microscope, numerous minute openings are seen. Around these are arranged rings of bone, with little black bodies in them, from which radiate fine, dark lines. These openings are sections of canals called Haversian canals, after Havers, an English physician, who first discovered them. The black bodies are minute cavities called lacunæ, while the fine lines are very minute canals, canaliculi, which connect the lacunæ and the Haversian canals. These Haversian canals are supplied with tiny blood-vessels, while the lacunæ contain bone cells. Very fine branches from these cells pass into the canaliculi. The Haversian canals run lengthwise of the bone; hence if the bone be divided longitudinally these canals will be opened along their length (Fig. 13).
Thus bones are not dry, lifeless substances, but are the very type of activity and change. In life they are richly supplied with blood from the nutrient artery and from the periosteum, by an endless network of nourishing canals throughout their whole structure. Bone has, therefore, like all other living structures, a self-formative power, and draws from the blood the materials for its own nutrition.
Fig. 13.
A, longitudinal section of bone, by which the Haversian canals are seen branching and communicating with one another;
B, cross section of a very thin slice of bone, magnified about 300 diameters--little openings (Haversian canals) are seen, and around them are ranged rings of bones with little black bodies (lacunæ), from which branch out fine dark lines (canaliculi);
C, a bone cell, highly magnified, lying in lacuna.
The Bones of the Head.
31. The Head, or Skull. The bones of the skeleton, the bony framework of our bodies, may be divided into those of the head, the trunk, and the limbs.
The bones of the head are described in two parts,--those of the cranium, or brain-case, and those of the face. Taken together, they form the skull. The head is usually said to contain 22 bones, of which 8 belong to the cranium and 14 to the face. In early childhood, the bones of the head are separate to allow the brain to expand; but as we grow older they gradually unite, the better to protect the delicate brain tissue.
32. The Cranium. The cranium is a dome-like structure, made up in the adult of 8 distinct bones firmly locked together. These bones are:
One Frontal,
Two Parietal,
Two Temporal
One Occipital,
One Sphenoid,
One Ethmoid.
The frontal bone forms the forehead and front of the head. It is united with the two parietal bones behind, and extends over the forehead to make the roofs of the sockets of the eyes. It is this bone which, in many races of man, gives a dignity of person and a beauty of form seen in no other animal.
The parietal bones form the sides and roof of the skull. They are bounded anteriorly by the frontal bone, posteriorly by the occipital, and laterally by the temporal and sphenoid bones. The two bones make a beautiful arch to aid in the protection of the brain.
The temporal bones, forming the temples on either side, are attached to the sphenoid bone in front, the parietals above, and the occipital behind. In each temporal bone is the cavity containing the organs of hearing. These bones are so called because the hair usually first turns gray over them.
The occipital bone forms the lower part of the base of the skull, as well as the back of the head. It is a broad, curved bone, and rests on the topmost vertebra (atlas) of the backbone; its lower part is pierced by a large oval opening called the foramen magnum, through which the spinal cord passes from the brain (Fig. 15).
The sphenoid bone is in front of the occipital, forming a part of the base of the skull. It is wedged between the bones of the face and those of the cranium, and locks together fourteen different bones. It bears a remarkable resemblance to a bat with extended wings, and forms a series of girders to the arches of the cranium.
The ethmoid bone is situated between the bones of the cranium and those of the face, just at the root of the nose. It forms a part of the floor of the cranium. It is a delicate, spongy bone, and is so called because it is perforated with numerous holes like a sieve, through which the nerves of smell pass from the brain to the nose.
Fig. 14.--The Skull
33. The Face. The bones of the face serve, to a marked extent, in giving form and expression to the human countenance. Upon these bones depend, in a measure, the build of the forehead, the shape of the chin, the size of the eyes, the prominence of the cheeks, the contour of the nose, and other marks which are reflected in the beauty or ugliness of the face.
The face is made up of fourteen bones which, with the exception of the lower jaw, are, like those of the cranium, closely interlocked with each other. By this union these bones help form a number of cavities which contain most important and vital organs. The two deep, cup-like sockets, called the orbits, contain the organs of sight. In the cavities of the nose is located the sense of smell, while the buccal cavity, or mouth, is the site of the sense of taste, and plays besides an important part in the first act of digestion and in the function of speech.
The bones of the face are:
Two Superior Maxillary,
Two Malar,
Two Nasal,
Two Lachrymal,
Two Palate,
Two Turbinated,
One Vomer,
One Lower Maxillary.
34. Bones of the Face. The superior maxillary or upper jawbones form a part of the roof of the mouth and the entire floor of the orbits. In them is fixed the upper set of teeth.
The malar or cheek bones are joined to the upper jawbones, and help form the sockets of the eyes. They send an arch backwards to join the temporal bones. These bones are remarkably thick and strong, and are specially adapted to resist the injury to which this part of the face is exposed.
The nasal or nose bones