All living bodies are made up of tissues. There is no part, no organ, however soft and yielding, or hard and resisting, which has not this peculiarity of structure. The bones of animals, as well as their flesh and fat, are composed of tissues, and all alike made up of cells. When viewed under a microscope, each cell is seen to consist of three distinct parts, a nucleolus, or dark spot, in the center of the cell, around which lies a mass of granules, called the nucleus; and this, in turn, is surrounded with a delicate, transparent membrane, termed the envelope. Each of the granules composing the nucleus assimilates nourishment, thereby growing into an independent cell, which possesses a triple organization similar to that of its parent, and in like manner reproduces other cells.

Illustration: Fig. 4.
Fig. 4. Nucleated cell. From Goeber. 1. Periphery of the cell, or cell-wall. 2. Nucleus. 3. Nucleolus in the center.

A variety of tissues enters into the composition of an animal structure, yet their differences are not always distinctly marked, since the characteristics of some are not unlike those of others. We shall notice, however, only the more important of the tissues.

The Areolar, or Connective Tissue, is a complete network of delicate fibers, spread over the body, and serves to bind the various organs and parts together. The fibrous and serous tissues are modifications of the areolar.

[pg 20]The Nervous Tissue is of two kinds: The gray, which is pulpy and granulated, and the white fibrous tissue. The Adipose Tissue is an extremely thin membrane, composed of closed cells which contain fat. It is found principally just beneath the skin, giving it a smooth, plump appearance.

Illustration: Fig. 5.
Fig. 5. Arrangement of fibers in the Areolar Tissue. Magnified 135 diameters.

The Cartilaginous Tissue consists of nucleated cells, and, with the exception of bone, is the hardest part of the animal frame. The Osseous Tissue, or bone, is more compact and solid than the cartilaginous, for it contains a greater quantity of lime. The Muscular Tissue is composed of bundles of fibers, which are enclosed in a cellular membrane.

Illustration: Fig. 6.
Fig. 6. Human Adipose Tissue.

Various opinions have been entertained in regard to the formation, or growth, of bone. Some anatomists have supposed that all bone is formed in cartilage. But this is not true, for there is an intra-membranous, as well as an intra-cartilaginous, formation of bone, as may be seen in the development of the cranial bones, where the gradual calcification takes place upon the inner layers of the fibrous coverings. Intra-cartilaginous deposit is found in the vicinity of the blood-vessels, within the cartilaginous canals; also, there are certain points first observed in the shafts of long bones, called centers of ossification. These points are no sooner formed than the cartilage corpuscles arrange themselves in concentric zones, and, lying in contact with one another, become very compact. As ossification proceeds, the cup-shaped cavities are converted into closed interstices of bone, with extremely thin lamellÆ, or layers. These, however, soon increase [pg 21]in density, and no blood-vessels can be observed within them.

Illustration: Fig. 7.
Fig. 7. Vertical section of cartilage near the surface of ossification. 1. Ordinary appearance of the temporary cartilage. 1'. Portion of the same more highly magnified. 2. The cells beginning to form into concentric zones. 2'. Portion more magnified. 3. The ossification is extending in the inter-cellular spaces, and the rows of cells are seen resting in the cavities so formed, the nuclei being more separated than above. 3'. Portion of the same more highly magnified.

Illustration: Fig. 8. Thigh-bone, sawn open lengthwise.
Fig. 8. Thigh-bone, sawn open lengthwise.

Illustration: Fig. 9. Lower end of the thigh-bone sawn across, showing its central cavity.
Fig. 9. Lower end of the thigh-bone sawn across, showing its central cavity.

The bony plates form the boundaries of the Haversian, or nutritive canals of the bones. In the second stage of ossification, the cartilage corpuscles are converted into bone. Becoming flattened against the osseous lamellÆ already formed, they crowd upon one another so as to entirely obliterate the lines that distinguish them; and, simultaneously with these changes, a calcareous deposit takes place upon their interior. Bones grow by additions to their ends and surfaces. In the child, their extremities are [pg 22]separated from the body of the bone by layer of cartilage, and the cancellated, or cellular structure, which remains for a time in the interior, represents the early condition of the ossifying substances.

The bones contain more earthy matter in their composition than any other part of the human body, being firm, hard, and of a lime color. They compose the skeleton or frame work, and, when united by natural ligaments, form what is known as the natural skeleton; when they are wired together, they are called an artificial skeleton. The number of bones in the human body is variously estimated; for those regarded as single by some anatomists are considered by others to consist of several distinct pieces. There are two hundred distinct bones in the human skeleton besides the teeth. These may be divided into those of the Head, Trunk, Upper Extremities, and Lower Extremities.

Illustration: Fig. 10. The bones of the skull separated.
Fig. 10. The bones of the skull separated. 1. Frontal, only half is seen. 2. Parietal. 3. Occipital, only half is seen. 4. Temporal. 5. Nasal. 6. Malar. 7. Superior maxillary (upper jaw). 8. Lachrymal. 9. Inferior maxillary (lower jaw). Between 4 and 6 a part of the sphenoid or wedge-shaped bone, is seen. Another bone assisting to form the skull, but not here seen, is called the ethmoid (sieve-like, from being full of holes), and is situated between the sockets of the eyes, forming the roof of the nose.

The Bones of the Head are classed as follows: eight belonging to the Cranium, and fourteen to the Face. The bones of the Cranium are the occipital, two parietal, two temporal, frontal, sphenoid, and ethmoid. Those composing the face are, the two nasal, two superior maxillary, two lachrymal, two malar two palate, two inferior turbinated, vomer, and inferior maxillary. The cranial bones are composed of two dense plates, between which there is, in most places a cancellated or cellular tissue. The external [pg 23]plate is fibrous, the internal, compact and vitreous. The skull is nearly oval in form, convex externally, the bone being much thicker at the base than elsewhere, and it is, in every respect admirably adapted to resist any injury to which it may be exposed, thus affording ample protection to the brain substance which it envelops. The internal surface of the cranium presents eminences and depressions for lodging the convolutions of the brain, and numerous furrows for the ramifications of the blood-vessels. The bones of the cranium are united to one another by ragged edges called sutures, which are quite distinct in the child but which in old age are nearly effaced. Some authorities suppose that by this arrangement the cranium is less liable to be fractured by blows; others think that the sutures allow the growth of these bones, which takes place by a gradual osseous enlargement at the margins. The bones of the Face are joined at the lower part and in front of the cranium, and serve for the attachment of powerful muscles which assist in the process of mastication. Although the soft parts of the face cover the bony structure, yet they do not conceal its principal features, or materially change its proportions. The form of the head and face presents some remarkable dissimilarities in different races.

Illustration: Fig. 11.
Fig. 11. 1. The first bone of the sternum (breast-bone). 2. The second bone of the sternum. 3. The cartilage of the sternum. 4. The first dorsal vertebra (a bone of the spinal column). 5. The last dorsal vertebra. 6. The first rib. 7. Its head. 8. Its neck. 9. Its tubercle. 10. The seventh or last true rib. 11. The cartilage of the third rib. 12. The floating ribs.

Illustration: Fig. 12.
Fig. 12. A vertebra of the neck. 1. The body of the vertebra. 2. The spinal canal. 4. The spinous process cleft at its extremity. 5. The transverse process. 7. The interior articular process. 8. The superior articular process.

The Trunk has fifty-four bones, which are as follows: The Os Hyoides, the Sternum, twenty-four Ribs, twenty-four vertebrÆ or bones of the Spinal Column, the Sacrum, the Coccyx, and two Ossa Innominata. The Os Hyoides, situated at the base of the tongue, is the most isolated bone of the skeleton, and serves for the attachment of [pg 24]muscles. The Sternum, or breast-bone, in a child is composed of six pieces, in the adult of three, which in old age are consolidated into one bone. The Ribs are thin, curved bones, being convex externally. There are twelve on each side, and all are attached to the spinal column. The seven upper ribs, which are united in front of the sternum, are termed true ribs; the next three, which are not attached to the sternum, but to one another are called false ribs; and the last two, which are joined only to the vertebrÆ, are designated as floating ribs. The first rib is the shortest, and they increase in length as far as the eighth, after which this order is reversed.

Illustration: Fig. 13.
Fig. 13. 1. The cartilaginous substance which connects the bodies of the vertebrÆ. 2. The body of the vertebra. 3. The spinous process. 4,4. The transverse processes. 5,5. The articular processes. 6,6. A portion of the bony bridge which assists in forming the spinal canal (7).

Illustration: Fig. 14.
Fig. 14. Backbone, spinal column, or vertebral column. All animals possessing such a row of bones are called vertebrates. Above b are the cervical (neck) vertebrÆ; b to c, dorsal (back) or chest vertebrÆ; c to d, lumbar (loins) vertebrÆ; d to e, sacrum; e to f, coccyx.

The Spinal Column or backbone, when viewed from the front presents a perpendicular appearance, but a side view shows four distinct curves. The bones composing it are called vertebrÆ. The body part of a vertebra is light and spongy in texture, having seven projections called processes, four of which are the articular processes, which furnish surfaces to join the different vertebrÆ of the spinal column. Two are called transverse, and the remaining one is termed the spinous. The transverse [pg 25]and spinous processes serve for the attachment of the muscles belonging to the back. All these processes are more compact than the body of the vertebra, and, when naturally connected, are so arranged as to form a tube which contains the medulla spinalis, or spinal cord. Between the vertebrÆ is a highly-elastic, cartilaginous and cushion-like substance, which freely admits of motion, and allows the spine to bend as occasion requires. The natural curvatures of the spinal column diminish the shock produced by falling, running or leaping, which would otherwise be more directly transmitted to the brain. The ribs at the sides, the sternum in front, and the twelve dorsal bones of the spinal column behind, bound the thoracic cavity, which contains the lungs, heart, and large blood-vessels.

Illustration: Fig. 15. A representation of the pelvic bones.
Fig. 15. A representation of the pelvic bones. e. The lumbo-sacral joint. 2. The sacrum. 3. Coccyx. 1, 1. The innominata. 4,4. Acetabula.

The Pelvis is an open bony structure, consisting of the Os Innominata, one on either side, and the Sacrum and Coccyx behind. The Sacrum, during childhood, consists of five bones, which in later years unite to form one bone. It is light and spongy in texture, and the upper surface articulates with the lowest vertebra, while it is united at its inferior margin to the coccyx. The Coccyx is the terminal bone of the spinal column. In infancy it is cartilaginous and composed of several pieces, but in the adult these unite and form one bone. The Innominata, or nameless bones, during youth, consist of three separate pieces on each side; but as age advances they coalesce and form one bone. A deep socket, called the acetabulum, is found near their junction, which serves for the reception of the head of the thigh-bone.

Illustration: Fig. 16.
Fig. 16. 1. Portions of the backbone. 2. Cranial bones. 4. Breast-bone. 5. Ribs. 7. Collar-bone. 8. Arm-bone (humerus). 9. Shoulder-joint. 10, 11. Bones of the fore-arm (ulna and radius). 12. Elbow-joint. 13. Wrist-joint. 14. Bones of the hand. 15, 16. Pelvic bones. 17. Hip-joint. 18. Femur. 19, 20. Bones of the knee-joint. 21, 22. Fibula and tibia. 23. Ankle bone. 24. Bones of the foot.

The Bones of the Upper Extremities are sixty-four in number, and are classified as follows: The Scapula, [pg 26]Clavicle, Humerus, Ulna, Radius, Carpus, Metacarpus, and Phalanges. The Scapula, or shoulder-blade, is an irregular, thin, triangular bone, situated at the posterior part of the shoulder, and attached to the upper and back part of the chest. The Clavicle, or collar-bone, is located at the upper part of the chest, between the sternum and scapula, and connects with both. Its form resembles that of the italic letter f, and it prevents the arms from sliding forward. The Humerus, the first bone of the arm, is long, cylindrical, and situated between the scapula and fore-arm. The Ulna is nearly parallel with the radius, and situated on the inner side of the fore-arm. It is the longer and larger of the two bones, and in its articulation with the humerus, forms a perfect hinge-joint. The Radius, so called from its resemblance to a spoke, is on the outer side of the fore-arm, and articulates with the bones of the wrist, forming a joint. The ulna and radius also articulate with each other at their extremities. The Carpus, or wrist, consists of eight bones, arranged in two rows. The Metacarpus, or palm of the hand, is composed of five bones [pg 27]situated between the carpus and fingers. The Phalanges, fourteen in number, are the bones of the fingers and thumb, the fingers each having three and the thumb two.

The Bones of the Lower Extremities, sixty in number, are classed as follows: The Femur, Patella, Tibia, Fibula, Tarsus, Metatarsus, and Phalanges. The Femur, or thigh-bone, is the longest bone in the body. It has a large round head, which is received into the acetabulum, thus affording a good illustration of a ball and socket joint. The Patella, or knee-pan, is the most complicated articulation of the body. It is of a round form, connects with the tibia by means of a strong ligament, and serves to protect the front of the joint, and to increase the leverage of the muscles attached to it, by causing them to act at a greater angle. The Tibia, or shin bone, is enlarged at each extremity and articulates with the femur above and the astragalus, the upper bone of the tarsus, below. The Fibula, the small bone of the leg, is situated on the outer side of the tibia, and is firmly bound to it at each extremity. The Tarsus, or instep, is composed of seven bones, and corresponds to the carpus of the upper extremities. The Metatarsus, the middle of the foot, bears a dose resemblance to the metacarpus, and consists of five bones situated between the tarsus and the phalanges. The tarsal and the metatarsal bones are so united as to give an arched appearance to the foot, thus imparting elasticity. The Phalanges, the toes, consist of fourteen bones, arranged in a manner similar to that of the fingers.

We are not less interested in tracing the formation of bone through its several stages, than in considering other parts of the human system. The formation of the Haversian canals for the passage of blood-vessels to nourish the bones, the earlier construction of bony tissue by a metamorphosis of cartilaginous substance, and also the commencement of ossification at distinct points, called centers of ossification, are all important subjects, requiring the student's careful attention. The bones are protected by an external membranous envelope, which, from its situation is called the periosteum. The bones are divided into four classes, long, short, flat and irregular, being thus adapted to subserve a variety of purposes.

The Long Bones are found in the limbs, where they act as [pg 28]levers to sustain the body and aid in locomotion. Eachlong bone is composed of a cylinder, known as the shaft, and two extremities. The shaft is hollow, its wails being thickest in the middle and growing thinner toward the extremities. The extremities are usually considerably enlarged, for convenience of connection with other bones, and to afford a broad surface for the attachment of muscles. The clavical, humerus, radius, ulna, femur, tibia, fibula, the bones of the metacarpus, metatarsus and the phalanges, are classed as long bones.

Where the principal object to be attained is strength, and the motion of the skeleton is limited, the individual bones are short and compressed, as the bones of the carpus and tarsus. The structure of these bones is spongy, except at the surface, where there is a thin crust of compact matter.

Illustration: Fig. 17. Bones of a joint.
Fig. 17. Anatomy of a joint, 1, 1. Bones of a joint. 2, 2. Cartilage. 3, 3, 3, 3. Synovial membrane.

Illustration: Fig. 18.
Fig. 18. Anatomy of knee joint. 1. Lower end of thigh-bone. 3. Knee-pan. 2, 4 Ligaments of the knee-pan. 5. Upper end of the tibia, or shin-bone. 6, 12. Cartilages.

When protection is required for the organs of the body, or a broad flat surface for the attachment of the muscles, the bones are expanded into plates, as in the cranium and shoulder-blades.

The irregular or mixed bones are those which, from their peculiar shape, cannot be classed among any of the foregoing divisions. Their structure is similar to the others, consisting of cancellar tissue, surrounded by a crust of compact matter.

The vertebrÆ, sacrum, coccyx, temporal, sphenoid, ethmoid, malar, two maxillary, palate, inferior turbinated, and hyoid are known as irregular bones.

The formation of the joints requires not only bones, but also [pg 29]cartilages, ligaments, and the synovial membrane, to complete the articulation. Cartilage is a smooth, elastic substance, softer than bone, and invested with a thin membrane, called perichondrium. When cartilage is placed upon convex surfaces, the reverse is true. The Ligaments are white, inelastic, tendinous substances, softer than cartilage, but harder than membrane. Their function is to bind together the bones. The Synovial Membrane covers the cartilages, and is then reflected upon the ligaments, thus forming a thin, closed sac, called the synovial capsule.

All the synovial membranes secrete a lubricating fluid, termed synovia, which enables the surfaces of the bones and ligaments to move freely upon one another. When this fluid is secreted in excessive quantities, it produces a disease known as "dropsy of the joints." There are numerous smaller sacs besides the synovial, called bursÆ mucosÆ, which in structure are analogous to them, and secrete a similar fluid. Some joints permit motion in every direction, as the shoulders, some in two directions only, as the elbows, while others do not admit of any movement. The bones, ligaments, cartilages, and synovial membrane, are supplied with nerves, arteries, and veins.

When an animal is provided with an internal bony structure, it indicates a high rank in the scale of organization. An elaborate texture of bone is found in no class below the vertebrates. Even in the lower order of this sub-kingdom, which is the highest of animals, bone does not exist, as is the case in some tribes of fishes, such as sharks, etc., and in all classes below that of the cartilaginous fishes, the inflexible substance which sustains the soft parts is either shell or some modification of bone, and is usually found on the outside of the body. True bone, on the contrary, is found in the interior, and, therefore, in higher animals, the skeleton is always internal, while the soft parts are placed external to the bony frame. While many animals of the lowest species, being composed of soft gelatinous matter, are buoyant in water, the highest type of animals requires not only a bony skeleton, but also a flexible, muscular system, for locomotion in the water or upon the land. Each species of the animal kingdom is thus organically adapted to its condition and sphere of life.

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