Anatomists talk about both bone andbones. The former is a type of connective tissue made up of cells suspended in a matrix: the collagenous matrix in bone just happens to beheavily impregnated with minerals. You will learn about bone cellselsewhere, but here is a picture of a cast of one, just to prove they exist . This osteocyte has characteristic long processes which runthrough the bone putting it in touch both with other cells and withblood vessels and nerves. Bones are discrete organs made up of bonetissue, plus a few other things.
The main misconception about bones then, is that they are made upof dead tissue. This is not true, they have cells, nerves, bloodvessels and pain receptors. Bone constituents, organic and inorganicmatrix and cells all turn over at a fairly rapid rate. If we treat abone with various solvents we can remove the inorganic matrix and leavethe flexible collagen. Or we can burn a bone and leave a hard brittleresidue.
The true structure of bone lies somewhere between these images. Intensile strength bone is rather like cast iron, although around 1/3 ofthe weight, in bending stress it behaves like steel, although only halfas strong and in compression it can withstand the forces exerted by arunning man (equivalent to a dead weight of 270kg). Even in standingthe compressive force on the hip joint, which you might expect to behalf the bodyweight on each side, is multiplied by a factor of aroundsix by muscular pull, since we are not in equilibrium when standing.
Determination of shape
The shape and structure ofbones is governed by many factors, genetic, metabolic and mechanical.Genetic determination of primary shape can be demonstrated by organculture of bone rudiments, which subsequently grow into recognisable bones, i.e. roughly the finished shape in all major respects. Fine tuning is by muscular action. The muscles are active in utero, althoughit is difficult to isolate their effect at this stage. After birth,however, and up to adolescence there is a correlation between activityand growth. this is seen in reverse if we look at people who arebedridden, or who have paralyses (such as poliomyelitis).
Metabolic factors are also important: calcium, phosphorous,vitamins A,C and D and the secretions of the pituitary, thyroid,parathyroid adrenals and gonads are all involved. Dwarves and giants arecontrolled by aberrant hormones, but there is much variation in normalheight. Absence of adequate supplies of vitamin D may lead to rickets,and absence of calcium in the diet to week bone liable to fracture.
Function
Origin of bone is again in two main forms. Some bone (in broadterms almost everything except the top of the skull) is preformed incartilage - replacement or endochondral bone. Details will come inhistology lectures. In the skull and one or two other places, however,bone forms direct in membranous connective tissue - membrane bone.
Look at the history of the skeleton to see why. Calcified skeletaltissues replaced silicacious in the Cambrian period, presumably becausephysiological changes either in the beasts or the oceans in which theylived allowed retention of Ca ions. Brachiopods, nautiloids, trilobitesgradually converted. Later the first vertebrates had bony scalesembedded in their skin - those around the mouth incidentally form the primitive basis of teeth. In some lines these scales fused to form bonycarapaces. These carapaces are retained over our heads as skull vaults.Later the rest of the skeleton, vertebrae etc., which were cartilaginous also became bony. This explains the distribution andorigins of membrane and cartilaginous bone. The surviving membranebones, notably in the head and part of the clavicle (a later inventionmade up of 2 fused bones, one membranous one cartilaginous) are bits ofdermal shield.
Whether in membrane or cartilage centres of ossification marked bythe appearance of calcified matrix appear over a long period of time,some in embryonic life, others in fetal and yet others well into the postnatal growing period. Many bones ossify from one centre, others froma group, of which one, the primary centre of ossification, is usuallycentral and early, and others, secondary centres, later and often peripheral.
Classification of bones
The skeleton is made upof many bones which change in proportion between man and his closerelatives but are easily recognisable. The easiest way to classifybones is by shape.
Long bones Typical of limbs, and a good place to start.They consist of a central, usually hollow, tubular region, thediaphysis linked to specialised ends (epiphysis) by a junctional region(metaphysis). Look at the shaft first. Tubular, a bit like a bicycleframe tube. Galileo was the first to write sensibly about this, noting that a hollow tube was stronger, weight for weight than a solid rod, andthat the dimensions had to be related to body weight rather than area:so the bones of an elephant have to be proportionally broader than those of a man. In some bones we can see adaptations for specificforces. For example the wing bones of vultures and other large birdshave strengthening that makes them very like bridges: it is a soberingthought that the first vulture predates the first girder bridge by somemillions of years. The diaphysis has layers of bone arranged likeplywood for strength. The cavity is filled with bone marrow (red andactive in children, yellow, fatty and inactive in adults). The shaftwalls are made of compact hard bone, and thickest in the middle whereforces are greatest. If these forces are too great the shaft mayfracture. Young bones have less calcium and are pliable, so fractureraggedly and partially (greenstick): older bones will fracture transversely or spirally according to force applied. Fractures usuallyheal spontaneously, albeit rather slowly in some cases, but the brokensurfaces need to be manipulated into the right place and may need to beheld with casts, pins or wires.
Towards the ends of the shaft the marrow cavity tends to be widerand filled with trabecular bone, arranged along lines of force whichhas a skeletal function in its own right and supports the marrow.
The ends of the bone are specialised to allow growth with as littleloss of strength as possible. The epiphysis permits this, but lookscomplicated. Lets go back to the fish long bone and try to understandthe logic underlying the structure. Growth must occur both at the endof the shaft (A) and over the surface of the joint (b). The fish accomplishes this by ending the bone with a simple plug of cartilage.This works until the bone has to bear weight, when the large, floppycartilage becomes an embarrassment. The first modification, seen in the chelonia (turtles and tortoises) is that the surface becomes curved(marrow cavities first seen in amphibians). The new radially arrangedcartilage is still strong but less bulky. The next problem is when theend is no longer hemispherical: the structure is then less strong, andcontinual reshaping is necessary with growth. These problems can beovercome by producing a secondary centre of ossification, already mentioned, - forming another lump of strong bone in the epiphysis. Thisis popular, occurring in at least seven groups of land vertebrates. Wesee secondary centres at the ends of most long bones, often more than one per end if the shape is complex. These ultimately fuse with the mainshaft in a process known as closure of the epiphysis. Which strengthensthe bone but ends the possibility of growth. This is a useful for forensic/medical purposes, but does not occur in a very regular way.
Short bones
Short bones are found in the wristand ankle, carpals and tarsals respectively. They have no shaft, as they do not increase dramatically in size in one dimension during growth,and tend to be cuboidal in shape. They are rather like a Malteser inconstruction, with cancellous bone in the centre and a hard outer shellof compact bone.
Flat bones
Flat bones like thoseof the cranium or the scapula are sandwiches of spongy bone between twolayers of compact bone. They are usually curved, so we can refer to aninner and outer table with diploe between them. These diploe,especially in the skull, may become pneumatised, i.e. filled with air. Aring of facial sinuses around the nose may become infected, leading tosinusitis.
Irregular bones
Any bones which don't fit thesearbitrary categories (bones of the face, vertebrae) are referred to as irregular.
Sesamoid
Sesamoid bones are interesting because theyoccur in tendon, especially where a tendon turns a corner, and is thusexposed to friction. We shall come across these again when we talk aboutmuscles.
Surface markings of bone.
We can often gleanclues about what is going on around a bone from its surface. In places,like joint surfaces, the bone will be covered with smooth articularcartilage. This falls off in preparation but leaves the underlying bonesmooth too. Bone is constantly growing or being reshaped, and this takesplace on the surface. At high magnification we can see, in a driedbone, what it was up to the point of death. This picture shows a holefor a blood vessel, a foramen. Around roughly half its diameter thecollagenous bone is rough, the other half smooth. The rough isresorbing bone, being eaten by large osteoclasts which leave pits andthe smooth is depositional, bone being formed. This indicates that theforamen was on the move as the bone grew. Other areas also showdeposition and resorption: these would be building up and hollowing outrespectively. On a macroscopic scale these effects can be seen as pointsof attachment to the bone - of ligaments, tendons or the fibrousinsertions of muscles. All these structures transmit forces, and demanda well organised junction. Any part of this structure which hasdeposited calcium will appear as a bit of bone. Within the bone weoften see rows of trabeculae or thick ropes of collagen, Sharpey'sfibres running across the marrow cavity to insert in the cortical boneopposite. Blood vessels and nerves similarly have canals.
The various lumps for fixing things to have different namesaccording to shape, usually derived from a dead language. There arelots of these, but common ones are:
I obtained this from a great page concerning Human Biology
Click here to visit: Human Biology Course Notes