Читать книгу Anatomy and Embalming - Charles Otto Dhonau - Страница 39

—Bone results from the calcification of cartilage or fibrous tissue. It is a highly specialized form of connective tissue. There are two varieties of bone; dense or compact bone and cancellous, loose, or spongy bone. Compact bone is dense, like ivory, and is always found on the exterior of bones.

Cancellous bone is found in the interior of bones, and has a lattice-work appearance.

Bone consists of one-third animal or organic matter and two-thirds earthy or inorganic matter. These proportions, however, vary with age. In youth it is nearly half and half, while in the adult the earthy is greatly in excess. It also varies with disease. With some defect of nutrition, the bone is deprived of its normal proportion of earthy matter, while the animal matter is of unhealthy quality, and we have as a result, a disease called rickets, so common in the children of the poor. The earthy or inorganic matter consists of phosphate, carbonate, fluoride of calcium, sodium chloride, and phosphate of magnesium. The animal matter consists of fat collagen, which when boiled with water is resolved into gelatin.

To illustrate the two substances, take a bone and place it in dilute hydrochloric acid. The acid will eat out all the mineral matter and we have left only the animal matter. After this operation one can take the bone and can bend it into any position whatever, which experiment shows that the animal matter gives elasticity to the bone.

The second experiment would be to put the bone on a bed of hot coals and burn it. Only the animal matter will burn and we will have the mineral matter remaining. After this operation one will find that the bone is very brittle and will easily break, which experiment shows that the mineral matter gives stability and support to the bone.

Fig. 5—Cross section of bone. (Sharpey)

If a cross section is made of any long bone, such as the humerus, and this section placed under the low power of the microscope, the Haversian canal system can be discerned. The Haversian canal system consists of the numerous small openings or canals through which the blood vessels ramify in distributing the nourishment to the bone. Around each individual canal are seen smaller spaces arranged in a circle. These are known as the lacunae (small lakes). Going from the lacunae are smaller canals which take on the name canaliculae, and joining all the lacunae together, making the appearance of concentric circles, we have the lamellae. The outside covering of the bone is called the periosteum and the inside covering is called the endosteum. Most of the long bones and many of the smaller bones are supplied by a nutrient artery, which enters the bone near its center, enters the bone marrow, and divides into two branches, one going up and the other down in the marrow. The blood is then distributed through the Haversian canal system. Veins emerge from the long bones in three places: 1. One or two large veins accompany the nutrient artery. 2. Numerous veins emerge from the articular extremities. 3. Many small veins arise in and emerge from the compact substance.

Bones are divided, according to shape, into four classes: long, short, flat and irregular.

Long Bones.—These bones are usually used as a system of levers to confer the power of locomotion. A long bone consists of a shaft and two extremities. The shaft is a hollow cylinder within which is the medullary canal. The extremities are somewhat expanded for the purpose of articulation, and to afford a broad surface for the attachment of muscles. The long bones are as a rule curved in two directions to give greater strength to the bone. Some examples of this class of bone are the clavicle, radius, ulna, humerus, femur, tibia, fibula, metacarpal, metatarsal, and the phalanges.

Short Bones.—These bones are placed in that part of the skeleton where there is need for strength and compactness, and where the motion of the part is slight and limited. Some examples of this class of bone are the bones of the carpus and tarsus (in the hand and the foot).

Flat Bones.—Flat bones are found where the principle requirement is either extensive protection, or the need of a broad surface for the attachment of muscles. Some of the bones of this class are the occipital, parietal, frontal, nasal, lachrymal, vomer, scapula, sternum, and the ribs.

Irregular Bones.—These bones are such as from their peculiar shape and form can not be grouped under any of the preceding heads. Some of the bones of this class are the vertebrae, sacrum, coccyx, temporal, sphenoid, ethmoid, etc.

If the surface of a bone is examined, certain articular and non-articular eminences and depressions will be seen.

Articular Eminences.—Examples of this class are found in the heads of the humerus and the femur.

Articular Depressions.—Examples of this class are found in the glenoid cavity of the scapula and the acetabulum.

Non-articular Eminences.—These are designated according to their form.

A tuberosity is a broad, rough, and uneven elevation.

A tubercle is a small, rough prominence.

A spine is a sharp, slender, pointed eminence.

A ridge, line, or crest is a narrow, rough elevation, running some way along the surface.

Non-articular Depressions.—These are of variable form, and are described as notches, sulci, fossae, grooves, furrows, fissures, etc. These non-articular eminences and depressions may serve to increase the extent of surface for the attachment of ligaments and muscles or may receive blood vessels, nerves, tendons, ligaments, or portions of organs.

Canals or foramina are channels or openings in bone through which pass the nerves and blood vessels.