Part 4

The flat bones of the cranial vault and sides are composed of an outer layer of ordinary compact substance, an inner layer of very dense bone, the =tabula vitrea=, and between these a variable amount of spongy bone, here termed =diploë=.

The =periosteum= is the membrane which invests the outer surface of bone, except where it is covered with cartilage. It 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. The degree of vascularity conforms to the activity of the periosteum.

The =marrow= (Medulla ossium) occupies the interstices of the spongy bone and the medullary cavity of the long bones. There are two varieties in the adult—red and yellow. In the young subject there is only =red marrow= (Medulla ossium rubra), but later this is replaced in the medullary cavity by =yellow marrow= (Medulla ossium flava). The red marrow contains several types of characteristic cells and is a blood-forming substance, while the yellow is practically ordinary adipose tissue.

Since yellow marrow is formed by regressive changes in red marrow, including fatty infiltration and degeneration of the characteristic cells, we find transitional forms or stages in the process. In aged or badly nourished subjects the marrow may undergo gelatinous degeneration, resulting in the formation of gelatinous marrow.

=Vessels and Nerves.=—It is customary to recognize two sets of =arteries=—the =periosteal= and the =medullary=. The former ramify in the periosteum and give off innumerable small branches which enter minute openings (Volkmann’s canals) on the surface and reach the Haversian canals of the compact substance. Other branches enter the extremities of the long bones and supply the spongy bone and marrow in them. In the case of the larger bones—and especially the long bones—the large =medullary= or =nutrient artery= enters at the so-called =nutrient foramen= (Foramen nutricium), passes in a canal (Canalis nutricius) through the compact substance, and ramifies in the marrow; its branches anastomose with the central branches of the periosteal set. The larger =veins= of the spongy bone do not, as a rule, accompany the arteries, but emerge chiefly near the articular surfaces. Within the bone they are destitute of valves.

The =lymph vessels= form perivascular channels in the periosteum and the Haversian canals of the compact substance. Lymph-spaces exist at the periphery of the marrow.

The =nerves= appear to be distributed chiefly to the blood-vessels. Special nerve-endings (Vater-Pacini corpuscles) in the periosteum are to be regarded as sensory, and probably are concerned in mediating the muscle sense (Kopsch).

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=.

FIG. 3.—LEFT FEMUR OF YOUNG PIG, EXTERNAL VIEW, TO SHOW DIVISION OF A LONG BONE INTO SHAFT (s) AND EXTREMITIES.

Proximal extremity consists of two parts, head (h) and trochanter major (t. m.), which have separate centers of ossification. Distal extremity consists of trochlea (t) and condyles (c); e.l., epiphyseal cartilages; s.f., supracondyloid fossa. ]

In intramembranous development the process begins at a definite =center of ossification= where the cells (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 trabeculæ. The trabeculæ rapidly thicken and coalesce, forming a bony plate which is separated from the adjacent bones by persistent fibrous tissue. 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.

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 trabeculæ 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.

A typical long bone is developed from three primary centers of ossification, one for the diaphysis or shaft and one for each epiphysis or extremity. Many bones have secondary centers from which processes or apophyses develop.

The foregoing outline accounts for the growth of bones except in regard to length. Increase in length may be explained briefly as follows: Provision for continued ossification at either end of the diaphysis is made by a layer of actively growing cartilage—the =epiphyseal cartilage=—which intervenes between the diaphysis and the epiphysis. It is evident that so long as this cartilage persists and grows, new bone may continue to be formed at its expense, and increase of length is possible. When the epiphyseal cartilage ceases to grow, it undergoes ossification, the bone is consolidated, and no further increase in length is possible. This fusion takes place at fairly definite periods in the various bones, and it is of value to know the usual times at which it occurs in the larger bones of the limbs at least.

After the bones have reached their full size, the periosteum becomes relatively reduced and inactive so far as its osteogenic layer is concerned; the bone-forming function may be stimulated by various causes, as is well seen in the healing of fractures and the occurrence of bony enlargements.

CHEMICAL COMPOSITION OF BONE

Dried bone consists of =organic= and =inorganic= matter in the ratio of 1 ∶ 2 approximately. The animal matter gives toughness and elasticity, the mineral matter 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 it very fragile. Conversely, decalcification, while not affecting the form and size of the bone, renders it soft and pliable. The animal matter when boiled yields gelatin. The following table represents the composition in 100 parts of ox bone of average quality:

Gelatin 33.30 Phosphate of lime 57.35 Carbonate of lime 3.85 Phosphate of magnesia 2.05 Carbonate and chlorid of sodium 3.45 —————— 100.00

PHYSICAL PROPERTIES OF BONE

Fresh dead bone has a yellowish-white color; when macerated or boiled and bleached, it is white. The specific gravity of fresh compact bone is a little over 1.93. It is very hard and resistant to pressure; a 5-millimeter cube of compact bone of the ox will resist pressure up to 852 pounds, if the pressure be applied in the line of the lamellæ (Rauber). Its tensile strength is estimated to be nearly twice that of oak.

DESCRIPTIVE TERMS

The surfaces of the bones present a great variety of eminences and depressions, as well as perforations. The prominences and cavities may be articular, or non-articular, furnishing attachment to muscles, tendons, ligaments or fascia. A number of descriptive terms are used to designate these features, and the following are some of those in general use:

=Process= (Processus) is a general term for a prominence.

A =tuberosity= (Tuber, Tuberositas) is a large, rounded projection; a =tubercle= (Tuberculum) is a smaller one.

The term =trochanter= is applied to a few prominences, e. g., the trochanters of the femur.

A =spine= (Spina) or =spinous process= (Processus spinosus) is a pointed projection.

A =crest= (Crista) is a sharp ridge.