OSTEOLOGY
In the construction of the human body, it would appear essential, in the first place, to provide some dense and solid texture capable of giving support and attachment to the softer parts of the frame, and at the same time to protect in closed cavities the more important vital organs; and such a structure we find provided in the various bones, which form what is called the Skeleton. Bone is one of the hardest structures of the animal body; it possesses also a certain degree of toughness and elasticity. Its colour, in a fresh state, is of a pinkish white externally, and deep red within. Chemical analysis resolves bone into an organic , or animal, and an inorganic , or earthy material, intimately combined together; the animal matter giving to bone its elasticity and toughness, the earthy part its hardness and solidity. The animal constituent may be separated from the earthy, by steeping bone in a dilute solution of nitric or muriatic acid: by this process the earthy constituents are gradually dissolved out, leaving a. tough semi-transparent substance which retains, in every respect, the original form of the bone. This is often called cartilage, but differs from it in being softer, more flexible, and, when boiled under a high pressure, it is almost entirely resolved into gelatine. The earthy constituent may be obtained by subjecting a bone to strong heat in an open fire with free access of air. By these means, the animal matter is entirely consumed, the earthy part remaining as a white brittle substance still preserving the original shape of the bone.
The organic or animal constituent of bone, forms about one-third , or 33,3 per cent.; the inorganic or earthy matter, two-thirds , or 66,7 per cent.: as is seen in the subjoined analysis by Berzelius:
| Animal Matter | Gelatine and Blood-vessels | 33,30 |
| Phosphate of Lime | 51,04 | |
| Inorganic or Earthy Matter | Carbonate of Lime | 11,30 |
| Fluoride of Calcium | 02,00 | |
| Phosphate of Magnesia | 01,16 | |
| Soda and Chloride of Sodium | 01,20 |
The proportion between these two constituents varies at different periods of life, as is seen in the following table from Schreger:
| Child | Adult | Old Age | |
| Animal matter | 47,20 | 20,18 | 12,20 |
| Earthy matter | 48,48 | 74,84 | 84,10 |
There are facts of some practical interest, bearing upon the difference here seen in the amount of the two constituents of bone, at different periods of life. Thus, in the child, where the animal matter forms nearly one-half of the weight of the bone, it is not uncommon to find, after an injury happening to the bones, that they become bent, or only partially broken, from the large amount of flexible animal matter which they contain. Again, also in aged people, where the bones contain a large proportion of earthy matter, the animal matter at the same time being deficient in quantity and quality, the bones are more brittle, their elasticity is destroyed; and, hence, fracture takes place more readily. Some of the diseases, also, to which bones are liable, mainly depend on the disproportion between the two constituents of bone. Thus, in the disease called rickets, bo common in the children of scrofulous parents, the bones become bent and curved, either from the superincumbent weight of the body, or under the action of certain muscles. This depends upon some deficiency of the nutritive system, by which bone becomes minim its normal proportion of earthy matter, whilst the animal matter is of unhealthy quality. In the vertebra of a rickety subject, Dr. Bostock found in IOO parts 79,75 animal, and 20,25 earthy matter.
The relative proportions of the two constituents of bone are found to differ in different bones of the skeleton. Thus the petrous portion of the temporal bone contains a large proportion of earthy matter, the bones of the limbs contain more earthy matter than those of the trunk, and those of the upper extremity, a larger proportion than those of the lower.
On examining a section of any bone, it is seen to be composed of two kinds of tissue, one of which is dense and compact in texture like ivory; the other open, reticular, spongy, enclosing cancelli or spaces, and hence called spongy or cancellated tissue. The compact tissue is always placed on the exterior of a bone; the cancellous tissue is always internal. The relative quantity of these two kinds of tissue varies in different bones, and in different parts of the same bone, as strength or lightness is requisite.
Form of Sonet. The various mechanical purposes for which bones are employed in the animal economy require them to be of very different forme. All the scientific principles of Architecture and Dynamics are more or less exemplified in the construction of this part of the human body. The power of the arch in resisting superincumbent pressure is well exhibited in various parts of the skeleton, such as the human foot, and more especially in the vaulted roof of the cranium.
Bones Division
Bones are divisible into four classes: Long, Short, Flat, and Irregular.
The long bones are found chiefly in the limbs, where they form a system of levers, which have to sustain the weight of the trunk, and to confer extensive powers of locomotion. A long bone consists of a lengthened cylinder or shaft, and two extremities. The shaft is a hollow cylinder, the walls consisting of dense compact tissue of great thickness in the middle, and becoming thinner towards the extremities; the spongy tissue is scanty, and the bone is hollowed out in its interior to form the medullary canal. The extremities are generally somewhat expanded for greater convenience of mutual connexion, and for the purposes of articulation. Here the bone is made up of spongy tissue with only a thin coating of compact substance. The long bones are the clavicle, humerus, radius, ulna, femur, tibia, fibula, metacarpat, and metatarsal bones and the phalanget.
Short Bones. Where a part is intended for strength and compactness, and the motion at the same time slight and limited, it is divided into a number of small pieces united together by ligaments, and the separate bones are short and compressed, sucli su the bones of the carpus and tarsus. These bones, id their struc- ture, are spongy throughout, excepting at their surface, where there is a thin crust of compact substance.
Flat Bones. Where the principal requirement is either extensive protection, or the provision of broad surfaces for muscular attachment, we find the osseous structure remarkable for its slight thickness, becoming expanded into broad flat plates, as is seen in the bones of the skull and shoulder-blade. These bones are composed of two thin layers of compact tissue, enclosing a layer of cancellous tissue of variable thickness. In the cranial bones, these layers of compact tissue are familiarly known as the tables of the skull; the outer one is thick and tough, the inner one thinner, denser, and more brittle, and hence termed the vitreous table. The intervening cancellous tissue is called the diploe. The flat bones are the occipital, parietal, frontal, nasal, lachrymal, vomer, scapulce, and ossa inno-minata.
The Irregular or Mixed bones are such as, from their peculiar form, cannot be grouped under either of the preceding heads. Their structure is similar to that of other bones, consisting of nn external layer of compact, and of a spongy cancellous substance within. The irregular bones are the vertebras, sacrum, coccyx, temporal, sphenoid, ethmoid, superior maxillary, inferior maxillary, palate, inferior turbinated, and hyoid.
Vessels of Kane
The blood-vessels of bone are very numerous. Those of the compact tissue consist of a close and dense network of vessels, which ramify in a fibrous membrane termed the periosteum, which covers the entire surface of the bone in nearly overy part. From this membrane, vessels pass through all parts of the compact tissue, running through the canals which traverse its substance. The cancellous tissue is supplied in a similar way, but by a less numerous set of larger vessels, which, perforating the outer compact tissue, are distributed to the cavities of the spongy portion of the bone. In the long bones, numerous apertures may be seen at the ends near the articular surfaces, some of which give passage to the arteries referred to; but the greater number, and these are the largest of them, are for the veins of the cancellous tissue which run separately from the arteries. The medullary canal is supplied by one large artery (or sometimes more), which enters the bone at the nutritious foramen (situated, in most cases, near the centre of the shaft), and perforates obliquely the compact substance. This vessel, usually accompanied by one or two veins, sends branches upwards and downwards, to supply the medullary membrane, which lines the central cavity and the adjoining canals. The ramifications of this vessel anastomose with the arteries both of the cancellous and compact tissues. The veins of bone are large, very numerous, and run in tortuous canals in the cancellous texture, the sides of which are constructed of a thin lamella of bone, perforated here and there for the passage of branches from the adjacent cancelli. The veins thus enclosed and supported by the hard structure, have exceedingly thin coats; and when the bony structure is divided, they remain patulous, and do not contract in the canals in which they are contained. Hence the constant occurrence of purulent absorption after amputation, in those cases where the stump becomes inflamed, and the cancellous tissue is infiltrated and bathed in pus. Lymphatic vessels have been traced into the substance of bone. Nerves, also, accompany the nutritious arteries into their interior.
Development of Bone
From the peculiar uses to which bone is applied, in forming a hard skeleton or framework for the softer materials of the body, and in enclosing and protecting some of the more important vital organs, we find its development takes place at a very early period. Hence the parts that appear soonest in the embryo, are the vertebral column and the skull, the great central column, to which the other parts of the skeleton are appended. At an early period of embryonic life, the parts destined to become bone consist of a congeries of cells, which constitutes the simplest form of cartilage. This temporary cartilage, as it is termed, is an exact miniature of the bone which in due course is to take its place; and as the process of ossification is slow, and not completed until adult life, it increases in bulk by an interstitial development of new cells. The next step in this process is the ossification of the intercellular substance, and of the cells composing the cartilage. Ossification commences in the interior of the cartilage at certain points, called points or centres of ossification, from which it extends into the surrounding substance. The period of ossification varies much in different bones. It commences first in the clavicle, in which the primitive point appears during the fifth week; next in the lower jaw. The ribs also, and the long bones of the limbs, appear soon after. The number of ossific centres varies in different bones. In most of the short bones, it commences by a single point in the centre, and proceeds towards the circumference. In the long bones, there is a central point of ossification for the shaft or diaphysis; and one for each extremity, the epiphyses. That for the shaft is the first to appear; those for the extremities appear later. For a long period after birth, a thin layer of unossified cartilage remains between the diaphysis and epiphyses, until their growth is finally completed. Processes such as the trochanters that have separate centres of ossification, are called epiphyses previous to their union.
Growth of Bone
Increase in the length of a bone, is provided for by the development of new bone from either end of the shaft (diaphysis); and in the thickness, by the deposition of new matter upon the surface: but when growth is at an end, the epiphyses become solidly united to the ends of the diaphysis, and the bone is completely formed. A knowledge of the exact periods when the epiphyses become joined to the shaft, aids the surgeon in the diagnosis of many of the injuries to which the joints are liable; for it not unfrequently happens, that on the application of severe force to a joint, the epiphyses become separated from the shaft, and such injuries may be mistaken for fracture.
The order in which the epiphyses become united to the shaft, follows a peculiar law, which appears to be regulated by the direction of the nutritious artery of the bone. Thus the arteries of the bones of the arm and forearm converge towards the elbow, and the epiphyses of the bones forming this joint become united to the shaft before those at the opposite extremity. In the lower extremities, on the contrary, the nutritious arteries pass in a direction from the knee; that is, upwards in the femur, downwards in the tibia and fibula; and in them it is observed, that the upper epiphysis of the femur, and the lower epiphyses of the tibia and fibula, become first united to the shaft.
| Cranium | 8 |
| Ossicula auditus | 6 |
| Face | 206 |
| Vertebral column (sacrum and coccyx included) | 14 |
| Os hyoides, sternum, and ribs | 26 |
| Upper extremities | 64 |
| Lower extremities | 62 |
A diseased condition of any joint makes considerable variation in the period of development of the several bones which enter into its formation. Thus, in chronic inflammation occurring in a joint at an early period of life, the epiphysal cartilages take on premature ossification; this process proceeding so rapidly, that it speedily becomes converted into bone, which becomes united to the shaft, and the bone ever after is considerably diminished in length: hence partial atrophy of the limb is the result.
