What is skeletal system ? Gross anatomy | Anjani Mishra

GENERAL OSTEOLOGY
(Skeletal System) 

 Written By Anjani Mishra

OSTEOLOGY: is the division of systematic anatomy which deals with description of the skeleton (bones and cartilage).

SKELETON: The term skeleton is applied to the frame work of hard structures which supports and protects the soft tissues of animals. In the descriptive Anatomy of higher animals, it is usually restricted to the bones and cartilages, although the ligaments which bind these together might well be included.

  

Classification
                                                                        Skeleton

                                                                 

A.    Appendicular                         B. Axial                                       C. Visceral

	Include the bones of limbs; forelimb or thoracic limb, hindlimb or pelvic limb		Includes the bones of head, vertebral column, ribs and sternum		Includes certain bones developed in the substance of some of the viscera or soft organs

 

 

Skeletal formula – Express the number of total bones in the skeleton.

 Skeleton formula of domestic animals and fowl:

 

Region	Cattle	Horse	Dog	Pig	Fowl
Vertebral column	50	54	52	52	41
Ribs and sternum	27	37	27	31	15
Forelimb	50	40	80	80	28
Hindlimb	50	40	76	80	42
Skull	34	34	34	30	40
Viscreal bones	2	--	1	1	1
Total	211	205	270	274	167

Bone: is one of the hardest connective tissue of the body which is suited for its supportive and protective function in the skeleton.

 Classification: The bones (ossa) are commonly divided into four classes according to their shape and function.This classification is not entirely satisfactory; some bones, e.g., the ribs, are not clearly provided for, and other might be variously placed.

1. Long bones (ossa longa):  Long bones are typically of elongated cylindrical form with enlarged extrimities. They occur in the limbs, where they act as supporting columns and as levers. The cylindrical part, termed the shaft or body (corpus), is tubular and encloses the medullary cavity (cavum medullare), which contains the medulla or marrow.

2. Flat bones (ossa plana):  Flat bones are expanded in two directions. They furnish sufficient area for the attachment of muscles and afford protection to the organs which they cover. e.g., scapula and many bones of the skull. Flat bones consists of two layers of compact bone with intervening spongy bone and marrow. The spongy layer in the bones of the skull is termed diploe.

3.  Short bones (ossa brevia): Short bones present somewhat similar dimensions in length, breadth and thickness. Their chief function appears to be that of diffusing concussion. e.g., carpus, tarsus and sesamoid bones. They diminish friction or change the direction of tendons  or increase leverage to muscles and tendons.

4. IRREGULAR BONES: This group include the bones of irregular shape and they are median and unpaired. Their functions are various and not so clearly specialized as those of the preceding classes. e.g., vertebrae and the bones of the cranial base.

     Functions:

              

·         Supports and protects the soft tissues of the body.

·         Plays vital role in the motility of the animal.

·         Important source of essential minerals like calcium and phosphorus.

·         Important source of blood cells like RBC, WBC, & platelets.

·         Provide body shape and bears the weight of animal body.

                                  

  Structure of bone

The architecture of bone can be studied best by means of longitudinal and transverse sections of specimens which have been macerated so as to remove most of the organic matter. These show that the bone consists of an external shell of dense compact substance, within which is the more loosely arranged spongy substance. In typical long bones the shaft is hollowed to form the medullary cavity.

Major constituents of bone

1. Periosteum
2. Compact bone
3. Spongy bone
4. Endosteum
5. The marrow

1) Periosteum – is the membrane which invest the outer surface of bone, except where it is covered with cartilage. It is a layer of specialized connective tissue. A periosteum layer is lacking on those areas of the epiphyses of long bones that are covered with articular cartilage. The periosteum consists of an outer protective fibrous layer and an inner cellular osteogenic layer. During active growth the osteogenic layer is well developed, but later it becomes much reduced. The fibrous layer varies much in thickness, being, in general, thickest in exposed situations. The adhesion of the periosteum to the bone also differs greatly in various places; it is usually very thin and easily detached where it is thickly Covered with muscular tissue which has little or no attachment.

2) Compact bone – differs greatly in thickness in various situations, in conformity with the stresses and strains to which the bone is subjected. In the long bones it is thickest in or near the middle part of the shaft and thins out toward the extremities. It is specially dense and smooth on joint surface. Circumscribed thickenings are found at points which are subject to special pressure or traction.

3) spongy bone – consists of delicate bony plates and spicules which runs in various directions and intercrosses. These are definitely arranged with regard to mechanical requirements, so that systems of pressure and tension plates can be recognized in conformity with the lines of pressure and the pull of tendons and ligaments respectively.The intervals between the plates are occupied lay marrow and are termed marrow spaces.The spongy substance forms the bulk of short bones and of the extremities of long bones; in the latter it is not confined to the ends but extends a variable distance along the shaft also.

Some bones contain air spaces within the compact substance instead of spongy bone and marrow and hence, are called pneumatic bones. The cavities are termed sinuses and are lined with mucous membrane; they communicate indirectly with the external air. In certain situations the two compact layers of flat bones are not separated by spongy bone, but fuse with each other; in some case of this kind the bone is so thin as to be translucent, or it may undergo absorption, producing on actual deficiency. The flat bones of the cranial vault and sides are composed of an outer layer of ordinary compact substance, the lamina externa, an inner layer of very dense bone, the lamina interna or tabula vitrea end between these a variable amount of spongy bone, here termed diploe.

4 ) endosteum – the endosteum is a thin fibrous membrane which lines the medullary cavity

and the larger haversian canals (nutrient canal of bone).

5) The marrow – occupies the interstices of the spongy bone and the medully cavity of the  long bones. There are two varieties in the adult - red and yellow. In the young subject there is only red narrow but later this is replaced in the medullay cavity by yellow marrow. The red marrow contains several types of characteristics cells and is a blood forming substance, where as the yellow is practically ordinary adipose tissue. Yellow marrow is formed by regressive changes in red marrow, including fatty infiltration and degeneration of the characteristics cells; thus we find transitional forms or stages in the process. In aged or badly nourished subjects the narrow may under go gelatinous degeneration, resulting in the formation of gelatinous marrow. Red narrow persists in the sternum through out the life and thus, this is a convenient place for examination and aspiration.

Development and Growth of bone

The primitive embryonal skeleton consists of cartilage and fibrous tissue, in which the bones develop. The process is termed ossification or osteogenesis, and is effected essentially by bone-producing cells, called osteoblasts. It is customary, therefore, to designate as membrane bones those which are developed in fibrous tissue, and as cartilage bones those which are preformed in cartilage. The principal membrane bones are those of the roof and sides of the cranium and most of the bones of the face. The cartilage bones comprise, therefore, most of the skeleton. Correspondingly we distinguish intramembranous and endochondral ossification. In intramembranous ossification the process begins at a definite center of ossification (Punctum ossificationis),where the osteoblasts surround themselves with a deposit of bone. The process extends from this center to the periphery of the future bone, thus producing a network of bony trabeculae. The trabeculae rapidly thicken and coalesce, forming a bony plate which is separated from the adjacent bones by persistent fibrous tissues.

The superficial part of the original tissue becomes periosteum, and on the deep face of this successive layers of periosteal bone are formed by osteoblasts until the bone attains its definitive thickness. Increase in circumference takes place by ossification of the surrounding fibrous tissue, which continues to grow until the bone has reached its definitive size. In endochondral ossification the process is fundamentally the same, but not quite so simple. Osteoblasts emigrate from the deep face of the perichondrium or primitive periosteum into the cartilage and cause calcification of the matrix or ground substance of the latter. Vessels extend into the calcifying area, the cartilage cells shrink and disappear, forming primary marrow cavities which are occupied by processes of the osteogenic tissue. There is thus formed a sort of scaffolding of calcareous trabeculae on which the bone is constructed by the osteoblasts. At the same time perichondral bone is formed by the osteoblasts of the primitive periosteum. The calcified cartilage is broken down and absorbed through the agency of large cells called osteoclasts, and is replaced by bone deposited by the osteoblasts. The osteoclasts also cause absorption of the primitive bone, producing the marrow cavities; thus in the case of the long bones the primitive central spongy bone is largely absorbed to form the medullary cavity of the shaft, and persists chiefly in the extremities. Destruction of the central part and formation of subperiosteal bone continue until the shaft of the bone has completed its growth.

Chemical and physical properties of bone

Dried  bone consists of organic and inorganic matter in the ratio of approximately 1:2. The animal matter gives toughness and elasticity, the mineral matter gives hardness to the bone tissue. Removal of the organic matter by heat does not change the general form of a bone, but reduces the weight by about one third and makes the bone very fragile. Conversely, decalcification, while not affecting the form and size of the bone, makes it soft and pliable. The organic matter (ossein) when boiled yields gelatin. The organic portion of bone consists chiefly of a protein, called bone collagen or ossein. Bone itself is a highly specialized form of C.T. that is hard and white and contains cells peculiar to it.

The hardness of bone is due to the deposition of mineral salts within the soft organic matrix. In addition to containing water bone consists of two main components;

             1.  The organic framework, and

       2.  The inorganic mineral salts (bone ash)

Between the collagenous fibres a fluid is found, resembling tissue fluid, an amorphous ground substance(mucopolysaccharides, hyaluronic acid, condroitin sulphate, keratin, and electrolytes). The following table represents the composition in 100 parts of ox bone of average quality:

Gelatin ----------------------------------------------------- 33.30

Calcium phosphate --------------------------------------- 57.35

Calcium carbonate ----------------------------------------  3.85

Magesium phosphate -------------------------------------  2.05  

Carbonate and chloride of sodium ----------------------  3.45

                                                                                   100.00

Fresh dead bone has a yellowish white color; when macerated or boiled and bleached, it is white. The specific gravity of fresh campact bone is about 1.9. It is very hard and resistent to pressure. Its compressive strength is about 20,000 pounds per square inch, and its tensile strength averages 15,000 pounds per square inch.

 composition of bone   

The hard extremely dense connective tissue that forms the skeleton of the body. It is composed of a matrix of collagen fibers impregnated with bone salts (chiefly calcium carbonate and calcium phosphate).

• Collagen fibers

by weight: 1/3 of bone

by volume: 1/2 of bone

• hydroxyapatite crystals

                 (ca)10(PO4)6(OH)2

                 95% solid (Vs. Water)

                 65% mineral; 35% Organic

Organic matter:

The growth and development of the bone is possible due to the organic matter contained in it. Organic matters are present in the bone in the form of membrane, cartilage, marrow, vessels, nerves and included fluids.

Inorganic matter:

Inorganic matters are present in the from of salts, deposited during the process of ossification of bone. The strength and hardness of a bone is due to the inorganic salt constituents.  

 

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