OSTEOLOGY. THE SKELETON. By ARTHUR THOMSON, F.R.C.S. Professor of Anatomy, University of Oxford. THE term skeleton (from the Greek, σkeλerós, dried) is applied to the parts which remain after the softer tissues of the body have been disintegrated or removed, and includes not only the bones, but also the cartilages and ligaments which bind them together. In the restricted sense of the word the skeleton denotes the osseous framework of the body. It is in this sense that it is generally employed in human anatomy. The skeleton serves to support the softer structures which are grouped around it, and also affords protection to many of the delicate organs which are lodged within its cavities. By the articulation of its several parts, its segments are converted into levers which constitute the passive portion of the locomotory system. Recent research has also proved that certain cells found in bone-marrow are intimately associated with the development and production of some of the corpuscles of the blood. Bone may be regarded as white fibrous tissue which, having become calcified has undergone subsequent changes, so as to be converted into true osseous tissue. Most probably all bone is of membranous origin, but it may pass through a stage in which cartilage plays an important part in its development. In many instances the cartilage persists, and is not converted into bone, as in the case of the articular cartilage which clothes the joint surfaces, the nasal septum, the cartilages of the nose, and the cartilages of the ribs. A persistence of the membranous condition is met with in man in the case of the tentorium cerebelli, which in some groups of animals (Carnivora) is converted into a bony partition. Skeletal structures may be derived from each of the three layers of the trilaminar blastoderm. The exo-skeleton includes structures of ectodermal, and some of mesodermal origin, in the shape of hair, nails, feathers, teeth, scales, armourplates, etc., whilst the endo-skeleton, with which we are more particularly concerned, is largely derived from the mesodermal tissue, but also includes the notochord, an entodermal structure which forms the primitive endo-skeleton, around which the axial skeleton is subsequently developed in the Vertebrata. The endo-skeleton is divisible into an axial portion, appertaining to the trunk and head, and an appendicular part, associated with the limbs. It also includes the splanchnic skeleton, which comprises certain bones developed in the substance of some of the viscera, such as the os cordis and os penis of certain mammals. In man, perhaps, the cartilaginous framework of the trachea and bronchi may be referred to this system. The number of the bones of the skeleton of man varies according to age. Owing to a process of fusion taking place during growth, the number in the adult is less than the number in the child. The following table does not include the sesamoid bones, which are frequently developed in tendons, the most constant ossicles of this description being those in relation to the metacarpo-phalangeal joint of the thumb, and the metatarso-phalangeal joint of the great toe. The table represents the number of bones distinct and separable during adult Bones are often classified according to their shape. Thus, long bones, that is to say, bones of elongated cylindrical form, are more or less characteristic of the limbs. Broad or flat bones are plate-like, and serve as protective coverings to the structures they overlie; the bones of the cranial vault display this particular form. Other bones, such as the carpus and tarsus, are termed short bones; whilst the bones of the cranial base, the face, and the vertebræ, are frequently referred to as irregular bones. Various descriptive terms are applied to the prominences commonly met with on a bone, such as tuberosity, eminence, protuberance, process, tubercle, spine, ridge, crest, and line. These may be articular in their nature, or may serve as points or lines of muscular and ligamentous attachment. The surface of the bone may be excavated into pits, depressions, foveæ, fossa, cavities, furrows, grooves, and notches. These may be articular or non-articular, the latter serving for the reception of organs, tendons, ligaments, vessels, and nerves. In some instances the substance of the bone is hollowed out to form an air space, sinus, or antrum. Bones are traversed by foramina and canals; these may be for the entrance and exit of nutrient vessels, or for the transmission of vessels and nerves from one region to another. A cleft, hiatus, or fissure serves the same purpose; channels of this kind are usually placed in the line of a suture, or correspond to the line of fusion of the primitive portions of the bone which they pierce. Composition of Bone.-Bone is composed of a combination of organic and inorganic substances in about the proportion of one to two. The animal matter may be removed by boiling or charring. According to the completeness with which the fibrous elements have been withdrawn, so the brittleness of the bone increases. When subjected to high temperatures the earthy matter alone remains. By soaking a bone in acid the salts may be dissolved out, leaving only the organic part. The shape of the bone is still retained, but the organic substance which is left is soft, and it can be bent about in any direction. The toughness and elasticity of bone depends therefore on its organic constituents, whilst its hardness is due to its mineral matter. Bone may be examined either in the fresh or dry condition. In the former state it retains all its organic parts, which include the fibrous tissue in and around it, the blood-vessels and their contents, together with the cellular elements found within the substance of the bone itself, and the marrow which occupies the lacunar spaces and marrow cavity. In the dried or macerated bone most of these have disappeared, though a considerable portion of the organic matter still remains, even in bones of great antiquity and in a more or less fossil condition. Considering its nature and the amount of material employed, bone possesses a remarkable strength, equal to nearly twice that of oak, whilst it is capable of resisting a greater crushing strain; it is stated that a cubic inch of bone will support a weight of over two tons. Its elasticity is remarkable, and is of the greatest service in enabling it to withstand the shocks to which it is so frequently subjected. In regions where wood is scarce the natives use the ribs of large mammals as a substitute in the construction of their bows. Its hardness and density vary in different parts of the skeleton, and its permanency and durability exceed that of any other tissue of the body, except the enamel and dentine of the teeth. The osseous remains of a race over eighty centuries old have been excavated in Egypt. Structure of Bone (Macroscopic).-To obtain an idea of the structure of a bone it is necessary to examine it both in the fresh or 'recent condition and in the macerated state. In the former the bone is covered by a membrane which is with difficulty torn off, owing to the abundance of fine fibrils which enter the substance of the bone from its deep surface. This membrane, called the periosteum, overlies the bone, except where the bone is coated with cartilage. This cartilage may form a bond of union between contiguous bones or, in the case of bones united to each other by movable joints, may be moulded into smooth articular surfaces called the articular cartilages. The attachment of the various ligaments and muscles can also be studied, and it will be noticed that where tendon or ligament is attached, the bone is often roughened to form a ridge or eminence; where fleshy muscular fibres are attached, the bone is, as a rule, smooth. In the macerated condition, when the cartilage and fibrous elements have been destroyed, it is possible, however, to determine with considerable accuracy the parts of the bone covered with articular cartilage, since the bone here is smooth and conforms generally to the curves of the articular areas of the joint; these areas are referred to as the articular surfaces of the bone. The bone, stripped of its periosteal covering, displays a dense surface finely pitted for the entrance of the processes derived from the periosteum, which thus establish a connexion between the bony substance and that vascular layer; here and there, more particularly in the neighbourhood of the articular extremities, these pits increase in size and number and allow of the transmission of small blood-vessels. If careful examination is made, one or two foramina of larger size will usually be noticed. These vascular foramina or canals allow the passage of arteries of considerable size into the interior of the bone, and are called the canales nutricii or nutrient canals or foramina of the bone. There are also corresponding channels for the escape of veins from the interior. In order more fully to ascertain the structure of bone it will be necessary to study it in section. Taking first a long bone, such as one meets with in the limbs, one notices on longitudinal section, that the bone is not of the same density throughout, for, whilst the external layers are solid and compact, the interior is made up of loose spongy bone called substantia spongiosa (cancellous tissue). Further, it will be observed that in certain situations this spongy substance is absent, so that there is a hollow in the interior of the bone called the medullary cavity. In the recent condition this cavity is filled with the marrow and is hence often called the marrow cavity. This marrow, which fills not only the marrow cavity but also the interstices between the fibres of the spongy substance, consists largely of fat cells, together with some marrow cells proper, supported by a kind of retiform tissue. The appearance and constituents of the marrow differ in different situations. In the medullary cavity of long bones the marrow, as above described, is known as medulla ossium flava (yellow marrow). In other situations, viz., in the diploë of the cranial bones (to be hereafter described), in the spongy tissue of such bones as the vertebræ, the sternum, and the ribs, the marrow is more fluid, less fatty, and is characterised by the presence of marrow-cells proper, which resemble in some respects colourless blood corpuscles. In addition to these, however, there are small reddish-coloured cells, akin to the nucleated red corpuscles of the blood of the embryo. These cells (erythroblasts) are concerned in the formation of the coloured corpuscles of the blood. Marrow which displays these characteristic appearances is distinguished from the yellow variety, already described, by being called the medulla ossium rubra (red marrow). The marrow met with in the spongy tissue of the cranial bones of aged individuals often undergoes degenerative changes and is sometimes referred to as gelatinous marrow. A better idea of the disposition of the bony framework of a long bone can be obtained by the examination of a section of a macerated specimen. In such a specimen the marrow has been destroyed and the osseous architecture of the bone is consequently better displayed. Within the body of the bone is seen the marrow cavity extending towards, but not reaching, either extremity of the bone. This cavity is surrounded on all sides by a loose spicular network of bone, which gradually increases in compactness until it reaches the circumference of the shaft, where it forms a dense surrounding wall. In the shaft of a long bone the thickness of this outer layer is not the same throughout, but tends to diminish as we approach the extremities, nor is it of uniform thickness on all sides of the bone. All the long bones display curves in varying degree, and it is a uniform rule that the thicker dense bone is found along the concave surface of the curve, thus assisting in materially strengthening the bone. Towards the extremities of the long bone the structure and arrangement of the bone undergoes a change. There is no marrow cavity, the spongy tissue is not so open and irregular, and the external wall is much thinner than in the shaft; indeed in many instances it is little thicker than stout paper. A closer examination of the arrangement of this spongy tissue throughout the bone suggests a regularity in its arrangement which might escape notice; and if, in place of one bone only being examined, sections of other bones are also inspected, it will be observed that the spicules of this tissue are so arranged as best to withstand the strains and stresses to which the bone is habitually subjected. From what has been said it will be obvious that the arrangements above described are those best adapted to secure the maximum of strength with the minimum of material, and a consequent reduction in the weight of the skeleton. The same description applies, with some modification, to bones of flattened form. Taking as an example the expanded plate-like bones of the cranial vault, their structure, as displayed on section, exhibits the following appearance: The outer and inner surfaces are formed by two compact and dense layers, having sandwiched between them a layer of spongy tissue called the diploë, containing red marrow. Note that there is no medullary cavity, though in certain situations and at certain periods of life the substance of the diploë may become absorbed and converted, by the evagination of the mucous membrane of the respiratory tract, into air-spaces or air-sinuses. Structure of Bone (Microscopic).—True bone differs from calcified cartilage or membrane in that it not merely consists of the deposition of earthy salts within its matrix, but displays a definite arrangement of its organic and inorganic parts. Compact bone merely differs from loose or spongy bone in the denseness of its tissue, the characteristic feature of which is the arrangement of the osseous lamellæ to form what are called Haversian systems. These consist of a central or Haversian canal, which contains the vessels of the bone. Around this the osseous lamellæ are arranged concentrically, separated here and there by interspaces called lacunæ, in which the bone corpuscles are lodged. Passing from these lacunæ are many fine channels called canaliculi. These are disposed radially to the Haversian canal, and pass through the osseous lamellæ. They are occupied by the slender processes of the bone corpuscles. Each Haversian system consists of from three to ten concentric rings of osseous lamellæ. In addition to the lamella of the Haversian systems there are others which are termed the interstitial lamellæ; these occupy the intervals between adjoining Haversian systems, and consist of Haversian systems which have undergone a process of partial absorption. Towards the surface of the bone, and subjacent to the periosteal membrane which surrounds the shaft, there are lamellæ arranged circumferentially; these are sometimes referred to as the outer fundamental lamellæ. The periosteal membrane which surrounds the bone, and which plays so important a part in its development, sends in processes through the various Haversian systems, which carry with them vessels and cells, thus forming an organic meshwork around which the earthy salts are deposited. -- Ossification of Bone. For an account of the earlier development of the skeleton the reader should consult a manual of embryology. Concerning the subsequent changes which take place, these are dependent on the conversion of the scleratogenous tissue into membrane and cartilage. A characteristic of this tissue. is that it contains elements which become formed into bone-producing cells, called osteoblasts. These are met with in the connective tissue from which the membrane bones are formed, whilst they also appear in the deeper layers of the investing tissue of the cartilage (perichondrium), and so lead to its conversion into the boneproducing layer or periosteum. All true bone, therefore, may probably be regarded as of membranous origin, though its appearance is preceded in some instances by the deposition of cartilage; in this case calcification of the cartilage is an essential stage in the process of bone formation, but the ultimate conversion into true bone, with characteristic Haversian systems, leads to the absorption and disappearance of this primitive calcified cartilage. In considering the development of bone an inspection of the skeleton of a foetus will enable the student to realise that much of what is bone in the adult is preformed in cartilage, whilst a part of the fully developed skeleton is represented only by membrane: hence, in regard to this ossification, bones have been described as of cartilaginous and membranous origin. If the development of a long bone is traced through successive stages from the cartilaginous condition in which it is preformed, it will be noticed that ossification begins in the body; the part of the bone ossified from this centre is referred to as the diaphysis, and, since it is the first to appear, the centre is spoken of as the primary centre of ossification. As yet, the ends of the body are cartilaginous knobs, but at a later stage one or more ossific centres appear in these cartilaginous extremities. These centres, which are independent of the diaphysis and appear much later, at variable periods, are termed secondary centres, and from them the epiphyses are formed. If there is more than one such centre at the end of a bone, the associated centres unite, and at a later stage the osseous mass so formed joins with the body or diaphysis, and in this way the formation of the bone is completed. Complete fusion by osseous union of the epiphyses with the diaphyses occurs at variable periods in the life of the individual. Prior to this taking place, the two are bonded together by a cartilaginous layer which marks the position of the epiphyseal line. If the bone is macerated at this stage of growth, the epiphysis falls away from the diaphysis. In the case of the articular ends of bone it will be noticed that the surfaces exposed by the separation of the epiphysis from the diaphysis are not plane and smooth, but often irregular, notched, and deeply pitted, so that when the two are brought together they interlock, and, as it were, dovetail into each other. In this way the extremities of the bone as yet ununited by osseous growth are, during youth and adolescence, able to withstand the shocks and jars to which during life they are habitually subjected. A long bone has been taken as the simplest example, but it by no means follows that these epiphyses are confined to the articular extremities of long bones. They are met with not only in relation to the articular surfaces of bones of varied form, but also occur where bones may be subjected to unusual pressure or to the strain of particular muscles. For this reason epiphyses of this nature have been called pressure and traction epiphyses (Parsons). There occur, however, secondary independent centres of ossification, which cannot be so accounted for. Possibly these are of phylogenetic interest only, and may accordingly be classed as Atavistic. Ossification in Membrane.-Membrane bones are such as have developed from fibrous tissue without having passed through a cartilaginous stage. Of this nature are the bones of the cranial vault and the majority of the bones of the face, viz., the maxillæ, zygomatic (malar), nasal, lacrimal, and palate bones, as well as the vomer. The medial lamina of the pterygoid process (internal pterygoid plate) is also of membranous origin. In the course of the development of a bone from membrane, as, for example, the parietal bone, the fibrous tissue corresponding to the position of the primary centre becomes osteogenetic, because here appear the bone-forming cells (osteoblasts), which rapidly surround themselves with a bony deposit more or less spicular in arrangement. As growth goes on these osteoblasts become embedded in the ossifying matrix, and remain as the corpuscles of the future bone, the spaces in which they are lodged corresponding to the lacunæ and canaliculi of the fully developed osseous tissue. From the primary centre ossifica |
