HUMAN ANATOMY – VOLUME 1

some tendons. These bones act as blocks, increasing the angle of attachment of tendons to bones, thus optimizing the force of muscle contraction.

Flat bones form walls of cavities and perform a protective function (bones of the skull, pelvis, sternum and ribs). They have significant surfaces for attachment of muscles.

poral, ethmoid, maxillary). The presence of cavities in these bones decreases the mass of the head. These cavities also act as voice resonators.

The surfaces of all bones have certain undulations on them, which correspond to places of attachment of muscles, fascies and ligaments. Eminencies, processes and tubera are called apophyses. Their formation is promoted by traction of muscle tendons. Places, where muscles attach to their fleshy part, are marked by recesses (pits, fossae). Along the periphery bones are bordered by edges. In places where vessels or nerves adjoin bones, their surfaces are marked with grooves or notches.

STRUCTURE AND CHEMICAL COMPOSITION OF BONES

Bones have a very specific place in the human organism. As any other organ, bones consist of different kinds of tissues, mainly, however, of osteal tissue, which is a variety of connective tissue.

Bones have a complex structure and chemical composition. In living organisms bones are 50 percent water, 28.5 percent organic substances and 21.85 percent inorganic material. The inorganic substances are compounds of calcium, magnesium, phosphorus and other elements. Macer-

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Fig. 38. Structure of tubular bone (acc. to V. Bargman).

1 — periosteum; 2 — compact bone; 3 — layer of external circumferential lamellaes; 4 — osteons; 5 — layer of internal circumferential lamellaes; 6 — marrow cavity; 7 — bone trabeculae of spongy bone.

4–15 mm thick, which are as if inserted one into another. One canal can be encircled by 4–20 such lamellae. The central canals with the surrounding lamellae are called osteons, or Haversian systems (Fig. 37). An osteon is a structural unit of compact substance of a bone (Fig. 38). The space between osteons is filled by intercalary lamellae. The external layer of compact bone is formed by outer lamellae. The internal layer, which limits the medullary cavity, is formed by inner lamellae.

Spongy (cancellous, trabecular) bone has the appearance of a sponge and is formed by bone trabeculae with spaces between them. The size and

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positioning of the trabeculae depends on the force exerted on the bone when it is stretched or compressed. Compression and strain curves are hypothetical lines that correspond to the orientation of the trabeculae (Fig. 39). Positioning of the trabeculae at an angle to each other results in more even distribution of pressure (from muscle traction) on the bone. This type of structure determines the durability of bones with a minimum of bone matter used.

means of which bone can grow in thickness and regenerate after injury. Thus, the periosteum performs protective, trophic and osteogenic functions.

Bone tissue possesses significant plasticity. It easily reconstructs itself after damage inflicted by training or physical loads. This manifests itself as an increase or reduction in the number of osteons, changes in thickness of lamellae of compact and spongy bone tissue. Optimal development of a bone takes place in conditions of moderate and regular physical loading. Sedentary lives with little physical loads redound to a weakening and thinning of bones. Spaces between lamellae become enlarged, the bone becomes porous and even partially resolves (bone resorption, osteoporosis). Professional activity, environmental and hereditary factors all influence the structure of bones.

Plasticity of bone tissue and its active reconstruction is realized through constant formation of new bone cells and extracellular matrix and parallel destruction (resorption) of old bone. Resorption is a result of osteoclast activity. In place of destroyed bone formation of new lamellae and osteons takes place.

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DEVELOPMENT AND GROWTH OF BONES

In its development the skeleton of a fetus passes through several stages, namely mesenchymal (connective tissue, membranous), cartilaginous and osseous. There are two ways of development of bone tissue, depending on the bone’s origin. Some bones form directly from embryonic connective tissue, skipping the cartilage stage. Bones of the vault of the skull, for example, are formed in this way (intramembranous ossification). Other bones pass through both membranous and cartilaginous stages. Bones of the trunk, limbs and base of the skull all develop from a cartilage model. In this case bone formation can be endochondral, perichondral and periosteal. Endochondral ossification takes place deep within cartilage; perichondral ossification takes place at the periphery of cartilage (with participation of the perichondrium). Ossification begins in one or several points inside the cartilage model. Around connective fibers and blood vessels that penetrate the cartilage young bone cells (osteoblasts) form trabeculae, which begin to increase in size and grow in different directions. Gradually, osteoblasts develop into mature osteocytes and bone tissue is formed.

Depending on the time period when bone tissue appears in the cartilage model it can be called a primary (main) or a secondary (accessory) center of ossification. Primary centers of ossification appear in diaphyses of tubular bones and most spongy and irregular bones during the first half of the prenatal period. Secondary ossification centers form in epiphyses of tubular bones at the end of prenatal development and after birth (until age of 17–18). These accessory centers of ossification provide for formation of processes, protuberances and crests.

A layer of cartilage (epiphyseal plate) remains between ossification centers of the diaphysis and epiphysis after their formation and is replaced by bone tissue only by age of 18–20. Growth of bone in thickness is promoted by the deep layer of the periosteum.

The medullary canal of tubular bones forms inside the diaphysis by means of resorption of endochondrally formed bone.

First signs of aging of bones manifest themselves as bone protrusions that appear at the periphery of articular surfaces. These are called marginal osteophytes. In hands they most typically appear on caputs of middle phalanges. As aging progresses they also appear in the base of the middle and distal phalanges. Diaphyses tend to widen due to an increase of periosteal osteogenesis. Growth and aging of bones depend on many factors, including the general state of the organism (lifestyle), as well as environmental influences.

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Questions for revision and examination

1.Name the anatomical formations /organs/ that pertain to the hard and the soft skeleton.

2.Name the functions carried out by the human skeleton.

3.What main characteristics is the classification of bones based on? Give examples.

4.What provides the durability of bones? Explain how and why bone durability changes in people of different ages.

5.What is an osteon and how is it formed?

6.Name the stages and ways of bone formation.

SKELETON OF THE TRUNK

The skeleton of the trunk is part of the axial skeleton. It consists of the vertebral column or spine and the thorax. The vertebral column (colúmna vertebrális) is formed by 33–34 vertebrae; among them there are 7 cervical, 12 thoracic and 5 lumbar vertebrae (Fig. 40). The 5 sacral vertebrae fuse to form one solid bone called sacrum (sacral bone). The coccyx consists of 3–5 coccygeal vertebrae.

VERTEBRAE

All vertebrae have a similar general structure independent of what part of the spine they belong to.

A vertebra (vértebra) consists of a body and an arch (Fig. 41). The v e r t e b r a l b o d y (córpus vértebrae) is situated to the front and serves as its supporting part. The v e r t e b r a l a r c h (árcus vértebrae) is connected to the body from behind with two pedicles. Between the body and the arch is the v e r t e b r a l f o r a m e n (forámen vertebrále). The combination of all these foramina forms the v e r t e b r a l c a n a l (canális vertebrális), which contains the spinal cord.

The posterior surface of the vertebral body has n u t r i e n t f o r a m i - n a though which nerves and blood vessels (arteries and veins) enter and leave the bone. On the vertebral arch there are processes, which serve as places of attachment for fasciae and muscles. The unpaired s p i n o u s p r o - c e s s (procéssus spinósus) protrudes backwards in the median plane, and to the left and right of the arch are the paired t r a n s v e r s e p r o c e s s e s (procéssus transvérsus). Upward and downward from the arch protrude the paired s u p e r i o r and i n f e r i o r a r t i c u l a r p r o c e s s e s (procéssus articuláres superióres et inferióres). At the bases of articular processes are the s u p e r i o r and i n f e r i o r v e r t e b r a l n o t c h e s (incisúrae vertebráles supérior et inférior). When two neighboring vertebrae are joined together, these notches form the right and left i n t e r v e r t e b r a l f o r a m i -

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and press against them. The articular processes of cervical vertebrae are relatively short. The spinous processes are short and bifurcated at the ends. The spinal process of the last cervical (C7) vertebra is longer and thicker than its neighboring analogues. It can be easily palpated and is called the vertebra prominence.

T h e f i r s t c e r v i c a l v e r t e b r a, t h e a t l a s (át l a s) does not have a body (Fig. 42). It is fused with the C2 vertebra during embryonic development, forming its dens. The atlas consists of the anterior and posterior arches, which are joined on the sides by lateral masses. Its

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Fig. 41. Structure of thoracic vertebra.

A — lateral aspect: 1 — vertebral body, 2 — superior costal facet; 3 — superior vertebral notch; 4 — superior articular process; 5 — transverse process; 6 — spinous process; 7 — inferior articular process; 8 — inferior vertebral notch; 9 — inferior costal facet. B — superior aspect: 1 — vertebral arch; 2 — transverse process; 3 — vertebral foramen; 4 — superior articular process; 5 — transverse process; 6 — spinous process.

vertebral foramen is large and round. On the front of the a n t e r i o r a r c h is the anterior tubercle. The inner surface of the arch has a recc — the facet for dens, which joins with the dens of the C2 vertebra. The p o s t e r i o r a r c h of the atlas has a p o s t e r i o r t u b e r - c l e, which is consid-

surfaces. The s u p e r i o r a r t i c u l a r s u r f a c e s have an oval shape; these connect with the condyles of the occipital bone. The i n f e r i o r a r t i c u l a r s u r f a c e s are round and participate in forming a joint with the second cervical vertebra. The upper surface of the posterior arch of the atlas has a g r o o v e f o r v e r t e b r a l a r t e r y on either side.

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Thoracic vertebrae

Thoracic vertebrae (vértebrae thorácicae) are larger than the cervical vertebrae. The height of their bodies increases in downward progression, with the T12 vertebra having the maximum height. Thoracic vertebrae (T2-T9) have s u p e r i o r and i n f e r i o r c o s t a l d e m i f a c e t s (fóvea costáles supérior et inférior). A superior costal demifacet of a lower vertebra combines with an inferior costal demifacet of an upper vertebra to form the articular facet for the head of the corresponding rib. T1, T10, T11 and T12 vertebrae have certain characteristic peculiarities. The T1 vertebra has full superior costal facets that articulate with heads of the first ribs, and inferior demifacets. These combine with the superior costal demifacets of the T2 vertebra to create full costal facets for the second ribs. T11 and T12 vertebrae have full costal facets for joints with the corresponding ribs.

The transverse processes of thoracic vertebrae are thickened on the ends. On the anterior sides of these processes are the transverse costal facets, which form costotransverse joints with the tubercles of ribs. T11 and T12 vertebrae do not have facets on their transverse processes. Spinous

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processes of thoracic vertebrae long and bent downwards lie overlapping each other. Such positioning prevents excessive bending of the spine. The articular processes are situated in the frontal plane, with the superior articular facets directed laterally and backward and the inferior articular facets — medially and forward.

Lumbar vertebrae

Lumbar vertebrae (vertebrae lumbales) have a large bean-shaped body, the height of which progressively increases from L1 to L5 vertebra. The vertebral foramina are large and almost triangular in shape. The transverse processes are situated nearly in the frontal plane. Spinous processes are flat, short, with thickened ends. Articular facets are directed medially on the superior articular processes and laterally on the inferior articular processes. On each superior articular process there is a very small tubercle called the m a m m i l l a r y p r o c e s s.

Sacrum

The sacrum (os sácrum) consists of five sacral vertebrae (vertebrae sacrales), which fuse into one bone during adolescence (Fig. 44). It is triangular in shape. The sacrum is a massive bone, as it takes on itself the weight of the whole body. The sacrum has a b a s e, an a p e x, a p e l v i c s u r f a c e, which faces forward, and a d o r s a l s u r f a c e. Articular processes on the base of the sacrum form a joint with the inferior articular processes of the L5 vertebra. The anterior edge of the base juts out in the form of a rounded angle called the p r o m o n t o r y. The concave pelvic surface has four t r a n s v e r s e l i n e s, which are traces of fusion between the bodies of the sacral vertebrae. At either side at the same level as these lines are the a n t e r i o r s a c r a l f o r a m i n a (forámina sacrália anterióra, s.pélvica). On the convex dorsal surface of the sacrum there are corresponding p o s t e r i o r s a c r a l f o r a m i n a (forámina sacrália posterióra, s.dorsália). There are five longitudinal crests where processes of sacral vertebrae have fused. The unpaired m i d i a n s a c r a l c r e s t is a result of fusion of the spinous processes. The paired i n t e r m e d i a t e

processes. On the upper lateral parts of the sacrum there are a u r i c u l a r s u r f a c e s (fácies auriculáres), which form joints with homonymous surfaces on the iliac bones. Between the auricular surfaces and lateral sacral crests there are tuberosities, to which ligaments and muscles attach. Vertebral foramina of the fused sacral vertebrae form the sacral canal, which ends in its lower part by the s a c r a l h i a t u s. On either side this hiatus is confined by the sacral horns, which are rudimental articular processes.

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