Anatomy of bone system. The manual for medical students / Анатомия костной системы. Учебное пособие для медицинских вузов. Г. И. Ничипорук
important role in reparative processes.
Almost all bones (except for most of the skull bones) have articular surfaces for junction with other bones. The articular surfaces are covered not with periosteum, but with articular cartilage, cartilago articularis. The articular cartilage has a specific, nonuniform structure: its superficial layer resembles a hyaline cartilage, the deep layer is fibrous.
The majority of bones have bone marrow inside – in spaces between lamellae of the spongy bone or in the medullary cavity, cavitas medullaris. The medullary cavity is covered inside with a specific sheath which is termed endosteum – endosteum. The endosteum, as well as the periosteum, plays a great role in metabolic processes in bones.
Bones of fetuses and newborns contain only red (haematogenic) bone marrow, medulla ossea rubra. It is a homogenous red color mass, rich in reticulate tissue, blood corpuscles and blood vessels. The total amount of red bone marrow is about 1500 cm3. In an adult, red bone marrow is partially substituted with yellow bone marrow, medulla osseum flava, which is mainly composed of adipose cells. Red bone marrow is substituted with yellow bone marrow only within medullar cavities.
1.2. Classification of Bones
It should be noted that there is no comprehensive classification of bones so far. For this purpose, various criteria are used in most of manuals on anatomy. At the same time, the principles of development and external structure features are often missed. Such feature as the structure of bones has an important clinical value. It determines the level of bone durability and specifications for treatment of injuries. In terms of phylogenesis, taking into consideration the existence of acranial and cranial organisms during the evolution, it is appropriate to divide bones into two groups: 1) bones of trunk and limbs; 2) skull bones. These bones differ from each other not only in their development but also in their structure.
According to the form and structure, four types of trunk and limb bones are distinguished: tubular, flat, volumetric and mixed bones.
Tubular bones have a cavity inside. They may be divided into long (humeral, forearm bones, femoral, leg bones, clavicle) and short (carpals, metatarsals, phalanges) bones.
In long tubular bones, one size prevails over other sizes. The middle part – diaphysis, diaphysis, (or body, corpus) of such bone has a cylindrical or triangular shape and consists of compact tissue, substantia compacta. Within the diaphysis, the medullary cavity is located. The bone ends – epiphyses, epiphyses, – are somewhat thickened. Their surfaces intended for joining with adjacent bones are covered with articular cartilage. On the inside, the epiphysises consist of spongy bone – substantia spongiosa, and on the outside there is a thin layer of compact bone – substantia compacta. Long tubular bones form the proximal and middle parts of the limb skeleton and play the role of leverages actuated by muscles. Short tubular bones form the distal parts of the limb skeleton and also consist of the middle part – the corpus and two ends called basis and caput.
Flat bones mainly consist of homogenous mass of spongy bone covered outside with a thin layer of compact bone. In flat bones, two sizes (width and length) prevail over 1.3. thickness. Such bones form the walls of cavities enclosing important organs, or represent extensive surfaces for attachment of muscles. Here belong the pelvic bones, sternum, scapulae and ribs.
Volumetric bones have the same structure, as the flat bones, i.e. they consist of a thin layer of compact bone outside and spongy bone inside. By shape, they resemble a cube with all dimensions roughly the same. Such bones are the carpal and tarsal bones. These bones are situated on the border between the middle and distal parts of the limbs, where not only high durability, but also high mobility is necessary.
Mixed bones are specific and complicated in shape. In their composition there are structural elements of volumetric and flat bones (vertebrae, sacrum, coccyx). Spongy tissue is contained in the bodies of these bones, and their other parts are mainly formed of compact bone. Such bones possess specific durability at continious loads.
Skull bones are classified by their location, development and structure.
According to location, they are divided into neurocranium bones and viscerocranium bones. According to development: into primary (endesmal), secondary (enchondral) and also mixed bones. The bones of the calvaria and viscerocranium are primary; the bones of the skull base are secondary; the occipital, sphenoidal and temporal bones are mixed; for example, the pyramid of the temporal bone and its mastoid part are secondary, but the squamous and tympanic parts of this bone are primary.
The skull bones have a very complicated external shape, thus it is appropriate to take their structure into consideration. According to the structure, it is possible to distinguish three types of the skull bones: 1) bones having diploё in their composition – diploic bones (parietal, occipital, frontal bones, mandible); 2) bones containing air cavities – pneumatized bones (temporal, sphenoid, ethmoid, frontal bones and maxilla); 3) bones built mainly from compact tissue – compact bones (lacrimal, zygomatic, palatine, nasal bones, inferior nasal concha, vomer, hyoid bone)The diploic substance is like spongy tissue, but its spaces between the osteal trabeculae are significantly smaller in diameter and have rounded shape.
1.3. Internal Structure of Bones
The internal structure of bones essentially differs in a fetus and in a newborn child. Therefore two types of osseous tissue are distinguished – reticulofibrous and lamellar. The reticulofibrous bone tissue is the basis of the embrional human skeleton. Its bony matrix is not arranged structurally, the bundles of collagen fibers are located in different directions, and they are directly connected with connective tissue surrounding the bone.
After birth, reticulofibrous tissue is replaced with lamellar tissue formed of osteal lamellae 4,5 – 11 mkm thick. There are osteal cells (osteocytes) in the smallest cavities (lacunae) between osteal lamellae. Collagen fibers in the bone lamellae are strictly arranged parallelly to their surfaces. They lose connection with the connective tissue surrounding the bone. They are connected with the periosteum only due to perforating (Sharpey`s) fibers running from the periosteum to the superficial layers of the bone. The lamellar bone is much more solid than the reticulofibrous bone. Substitution of one osseous tissue with another is caused by the influence of functional loads on the skeleton. On the section of a macerated bone (bone deprived of soft tissues), it is possible to see two types of the osseous tissue: compact and spongy. The compact bone, substantia compacta, is a solid bony mass located on the exterior of the bone. The osteal lamellae of the compact bone are very close to each other. The compact tissue coats the epiphyses of tubular and flat bones as a thin sheath. The diaphyses of tubular bones entirely consist of compact bone.
Spongy bone, substantia spongiosa, is formed by loosely located osteal lamellae. In the spaces between them there is red bone marrow. Spongy tissue forms epiphyses of tubular bones, bodies of vertebrae, ribs, sternum, pelvic bones and some hand and foot bones. Only the superficial cortical layer of such bones is comprised of the compact tissue. The spongy tissue of the skull bones has significantly smaller regularly shaped spaces in comparison with the trunk and limb bones. It has a specific name – diploё.
The structural and functional unit of the bone is osteon or Haversian system. It is possible to see osteons on thin sections or on histological preparations. The osteon is formed of concentrically arranged osteal (Haversian) lamellae which surround the Haversian canal in the form of cylinders of various sizes nested into each other. The Haversian canal contains blood vessels and nerves. The majority of osteons are oriented parallelly to the axis of the bone joining with each other in many points. The number of osteons is individual for each bone. For example, in the femoral bone this number is 1,8 per 1 mm2. Meanwhile, the share of the Haversian canal is 0,2 – 0,3 mm2. Between the osteons there are insert or intermediate lamellae which run in all directions. The intermediate lamellae are the remnants of destroyed old osteons. Processes of new formation and destruction of osteons continuously occur in bones.
There is a layer of internal circumferential lamellae, lamina circumferentialis interna in tubular bones on the border with the medullary cavity. They are permeated with numerous canals widening to spaces. Several