CHAPTER II.

OSTEOLOGY.

THE bones are the organs of support of the animal frame; they give firmness and strength to the entire fabric, afford points of connection to the numerous muscles, and bestow individual character on the body. In the limbs they are hollow cylinders, admirably calculated, by their conformation and structure, to resist violence and support weight. In the trunk and head, they are flattened and arched, to protect cavities and provide an extensive surface for attachment. In some situations they present projections of variable length, which serve as levers; and in others are grooved into smooth surfaces, which act as pulleys for the passage of tendons. Moreover, besides supplying strength and solidity, they are equally adapted, by their numerous divisions and mutual apposition, to fulfil every movement which may tend to the preservation of the creature, or be conducive to his welfare.

According to, the latest analysis by Berzelius, bone is composed of about onethird of animal substance, which is almost completely reducible to gelatine by boiling, and two-thirds of earthy and alkaline salts. The special constituents of bone are present in the following proportions:

[That bony tissue is composed essentially of two distinct elements, one organic and the other inorganic, can be readily shown, on the one hand, by calcination, when the whole of the organic matter is destroyed, emitting the odor of burned horn during the process, the residuary inorganic matter, which is light and porous, preserving the shape and size of the original bone, and being exceedingly friable, crumbles on the slightest pressure: whilst, on the other hand, if the bone be treated with dilute hydrochloric acid, the saline components are removed, and the remaining organic mass, which also preserves the original size and form of the bone, is tough and flexible, and much diminished in weight.]

Bones are divisible into three classes: - Long, flat, and irregular. The Long bones are found principally in the limbs, and consist of a shaft and two extremities. The shaft is cylindrical or prismoid in form, dense and hard in texture, and hollowed in the interior into a medullary canal. The extremities are broad and expanded, to articulate with adjoining bones; and, in internal structure, are cellular or cancellous. Upon the exterior of the bone are processes and rough surfaces for the attachment of muscles, and foramina for the transmission of vessels and nerves. The character of long bones is, therefore, their general type of structure and their divisibility into a central portion and extremities, and not so much their length; for there are certain long bones, as the second phalanges of the toes, which are less than a quarter of an inch in length, and which, in some instances, exceed in breadth their longitudinal axis. The long bones are, the clavicle, humerus, radius and ulna, femur, tibia and fibula, metacarpal bones, metatarsal, phalanges, and ribs.

Fiat bones are composed of two layers of dense bone with an intermediate cellular structure, and are divisible into surfaces, borders, angles, and processes.

They are adapted to inclose cavities; have processes upon their surface for the attachment of muscles; and are perforated by foramina, for the passage of nutrient vessels to their cells, and for the transmission of vessels and nerves. They articulate with long bones by means of smooth surfaces plated with cartilage, and with each other, either by fibro-cartilaginous tissue, as at the symphysis pubis, or by suture, as in the bones of the skull. The two condensed layers of the bones of the skull are named tables; and the intermediate cellular structure, diplöe. The flat bones are the occipital, parietal, frontal, nasal, lachrymal, vomer, sternum, scapulæ, and ossa innominata.

The Irregular bones include all that remain after the long and the flat bones have been selected. They are essentially irregular in their form, in some parts flat, in others short and thick; and are constructed on the same general principle as other bones: they have an exterior dense, and an interior more or less cellular. The bones of this class are the temporal, sphenoid, ethmoid, superior maxillary, inferior maxillary, palate, inferior turbinated, hyoid, vertebræ, sacrum, coccyx, carpal, tarsal, and sesamoid bones, the latter including the patellæ.

[The symmetry or want of symmetry of bones is also a basis for the determination of their figure. Thus, some bones being divisible into two halves, exactly resembling each other, are called symmetrical or azygos bones, and as these always occupy the mesial line, they are also denominated median; whilst the remainder, which cannot be divided into two similar parts, are called asymmetrical, and as they always occur in pairs, and are situated on opposite sides of the mesial line, they are also denominated corresponding or lateral bones.]

Structure of Bone.-Bone is a dense, compact, and homogeneous substance (basis substance) filled with minute cells [lacunæ] (corpuscles of Purkinje),

[FIG. 16.

FIG. 17.

MINUTE STRUCTURE OF BONE, drawn with the microscope from nature, by Bagg. Magnified 300 diameters. 1. One of the Haversian canals surrounded by its concentric lamellæ. The corpuscles are seen between the lamellæ; but the calcigerous tubuli are omitted. 2. An Haversian canal with its concentric lamellæ, Purkinjean corpuscles, and tubuli. 3. The area of one of the canals. 4, 4. Direction of the lamellæ of the great medullary canal. Between the lamella. at the upper part of the figure, several very long corpuscles with their tubuli are In the lower part of the figure, the outlines of two other canals are given, to show their form and mode of arrangement in the entire bone.

seen.

which are scattered numerously through its structure. The basis-substance of bone is subfibrous and obscurely lamellated, the lamellæ being concentric in long, and parallel in flat bones: it is traversed in all directions, but especially in the longitudinal axis, by branching and inosculating canals (Haversian' canals), which give passage to vessels and nerves, and in certain situations the lamella separate from each other, and leave between them areolar spaces (cancelli) of various magnitude. The lamellæ have an average diameter of ooo of an inch, and, besides constituting the general structure of the basis substance, are collected concentrically around the Haversian canals, and form boundaries to those canals of about of an inch in thickness. The number of lamellæ surrounding each Haversian canal is commonly ten or fifteen, and the diameter of the canals has a medium average of of an inch. The cancelli of bone, like its compact substance, have walls which are composed of lamella: and, such is the similarity in structure of the parts of a bone, that the entire bone may be compared to an Haversian canal, of which the medullary cavity is the magnified channel; and the Haversian canals may be likened to elongated and ramified cancelli. The Haversian canals are smallest near the surface of a bone, and largest near its centre, where they gradually merge into cancelli; by the frequent communications of their branches they form a coarse network in the basis substance.

TRANSVERSE SECTION OF A HUMAN FEMUR, about its middle, exhibiting the erratic course of the Haversian canals, and their relations to each other, and at the same the general laminated condition of a long bone. This laminated condition is well shown by polarized light, which causes the corpuscles to disappear, and the lamina to come out boldly.]

The corpuscles of Purkinje, are thickly disseminated through the basis-substance; they are irregular in size and form, give off numerous minute branching tubuli [canaliculi], which radiate from all parts of their circumference, and, in the dried state of the bone, contain merely the remains of membranous cells and some calcareous salts. In the living bone, the corpuscles and their tubuli are probably filled with a nutritive fluid holding calcareous salts in solution. The form of the cells is oval or round, and more or less flattened; their long diameter

[After their discoverer, Clopton Havers, an English physician and writer of the 17th century.]

Müller and Henle conceived that the bone cells and tubuli were the principal seat of the calcareous matter. Hence they were called calcigerous cells and tubuli.

corresponds with the long axis of the bone, and their tubuli cross the direction of the lamellæ, and constitute a delicate network in the basis-substance by communicating with each other and with the tubuli of neighboring cells. The tubuli of the cells nearest the Haversian canals terminate on the internal surface of

TWO LACUNE OF OSSEOUS TISSUE, seen on their surfaces, showing the disposition of their pores. The granular aspect of the tissue, both on their walls and around them, is well represented. Magnified 1200 diameters. Drawn from a preparation of the cancelli of the Femur made by Mr. Tomes.]

those passages. The size of the cells varies in extreme measurement from to of an inch in long diameter, an ordinary average being; the breadth of the oval cells is about one-half or one-third their length, and their thickness one-half their breadth. They are situated between the lamellæ, to which circumstance they owe their compressed form.

[FIG. 20.

THE PERIOSTEUM laid open and turned off from a young humerus.]

In the fresh state, bones are invested by a dense fibrous membrane, the periosteum, which covers every part of their surface with the exception of the articular extremities, the latter being coated by a thin layer of cartilage. The periosteum of the bones of the skull is termed pericranium; and the analogous membrane of external cartilages, perichondrium. Lining the interior of the medullary canal of long bones, the Haversian canals, the cancelli, and the cancelli of the flat and irregular bones, is the medullary membrane, which acts as an internal periosteum. It is through the medium of the vessels ramifying in these membranes that the changes required by nutrition occur in bones, and the secretion of medulla into their interior is effected. The medullary canal, Haversian canals and cancelli of long bones, and the cancelli of other bones, are filled with a yellowish, oily substance, the medulla, which is contained in a loose, cellular tissue formed by the medullary membrane.

Development of Bone. To explain the development of bone, it is necessary to inform the student that all organized bodies, whether belonging to the vegetable or animal kingdom, are developed primordially from minute vesicles. These vesicles, or, as they are properly termed, cells, are composed of a thin membrane, containing a fluid or granular matter, and a small rounded mass, the nucleus, around which the cell was originally formed. Moreover, the nucleus generally contains one or more small round granules, the nucleolus, or nucleoli. From cells having this structure all the tissues of the body are elaborated; the ovum itself originally presented this simple form, and the embryo at an early period is wholly composed of such nucleated cells. In their relation to each other, cells may be

isolated and independent, as is exemplified in the corpuscles of the blood, chyle, and lymph; secondly, they may cohere by their surfaces and borders, as in the epidermis and epithelium; thirdly, they may be connected by an intermediate substance, which is then termed intercellular, as in cartilage and bone; and, fourthly, they may unite with each other in rows, and upon the removal, by liquefaction, of their adherent surfaces, be converted into hollow tubuli. In the latter mode capillary vessels are formed, as also are the tubuli of nerves. One of the properties of cells may also be adverted to in this place; it is that of reproducing similar cells in their interior. In this case, the nucleoli become the nuclei of the secondary cells, and, as the latter increase in size, the membrane of the primary or parent cell is lost.

Bone, in its earliest stage, is composed of an assemblage of these minute cells, which are soft and transparent, and are disposed within the embryo in the site of the future skeleton. From the resemblance which the soft bone-tissue bears to jelly, this has been termed the gelatinous stage of osteo-genesis. As development advances, the cells, heretofore loosely connected together, become separated by the interposition of a transparent intercellular substance, which, at first fluid, gradually becomes hard and condensed. The cartilaginous stage of osteo-genesis is now established, and cartilage is shown to consist of a transparent basis-substance, having minute cells disseminated through it at pretty equal distances. Coincident with the formation of cartilage is the development of vascular canals in its substance, the canals being formed by the union of the cells in rows, and the subsequent liquefaction

of their adhering surfaces. The change which next ensues is the concentration of the vascular canals towards some one point; for example, the centre of the shaft in a long, or the mid-point of a flat bone, and here the punctum ossificationis, or centre of ossification, is established. What determines the vascular concentration now alluded to, is a question not easily solved, but that it takes place is certain, and the vascular punctum is the most easily demonstrable of all the phenomena of ossification.

FIGURES ILLUSTRATIVE OF THE DEVELOPMENT OF BONE;

A. A

magnified 155 times, and drawn with the camera lucida. portion of cartilage the furthest removed from the seat of ossification, showing simple nucleated cells, having an ordinary size of ' of an inch, long diameter. B. The same cartilage,nearer the seat of ossification; each simple cell has produced two, a little larger than the cells in figure a.

During the formation of the punctum ossificationis, changes begin to be apparent in the cartilage cells. Originally they are simple nucleated cells (3000 to zoo of an inch in diameter), having a rounded form. As growth proceeds, they become elongated, and it is then perceived that each cell contains. two and often three nucleoli, around which smaller cells are in progress of formation. If we examine them nearer the punctum ossificationis, we find that the young or secondary cells have each attained the size of the parent cell (zoo of an inch), the membrane of the parent cell has disappeared, and the young cells are separated to a short distance by freshly effused intercellular substance. Nearer still to the punctum ossificationis a more remarkable change has ensued, the energy of cellule reproduction has augmented with proximity to the ossifying point, and each cell, in place of producing two, gives birth to four, five, or six young cells, which rapidly destroy the parent membrane and attain a greater size (3 of an inch) than the parent cell, each cell being, as in the previous