Page:The American Cyclopædia (1879) Volume III.djvu/66

60 BONE soft organized substance, forming an inter- lacing network of bone corpuscles and fila- ments, destined to absorb nourishment from tbe blood vessels occupying the Haversian canals. The bone corpuscles have an average length of -,-gVir of an inch, and they are usually about one half as wide and one eighth as thick. The diameter of the pores, or canaliculi, is from TTT.iw to -nr.W of an inch.-From the re- searches of Mr. Tomes and Mr. Quekett it appears that the ultimate structure of bone consists of a congeries of granular parti- cles, deposited in an organized matrix; these granules are often distinctly visible, with- out any artificial preparation, in the sub- stance of the delicate spicula of the cancelli, varying in size from -^^ to Tr.^nr of an inch - The periosteum, a dense, fibrous membrane, richly supplied with blood vessels, covers the external surface of all bones, with the excep- tion of their articular extremities. The vessels of bone are supplied from the periosteum, and ramify, as has been seen, through the Haver- sian canals ; in the long bones a large artery penetrates by the nutritious foramen into the medullary cavity, sending branches to the med- ullary cells, and inosculating with the capil- laries from other sources. Nerves have not yet been detected in the interior of bones sup- plying strictly the osseous structure, but the painfulness of many diseases of the bones shows that the external and internal vascular surface must be supplied with nerves. Lymphatics most probably also exist in bone. At the ear- liest period of the appearance of a skeleton in the embryo, it consists of a series of cells ; these increase in number and density, and are held together by an intercellular substance, thus forming temporary cartilage, which is after- ward converted into bone, though not com- pletely so until adult age. Ossification com- mences at determinate points or centres, the first of which is in the clavicle, and appears during the fourth week ; then follow the lower jaw, ribs, femur, humerus, tibia, and upper jaw ; the spine and pelvis are late, and the kneepan does not begin to ossify till after birth. There are generally several ossific cen- tres ; for instance, in the long bones, one for the shaft, and one for each extremity. The cen- tral part of the bone is the diaphysis, and is not united till long after birth to the ends or epiphyses ; processes of bone are called apo- pkyses. Ossification generally extends in the intended direction of the chief strength of a bone. According to Todd and Bowman, the process by which cartilage is converted into bone is as follows : The small nucleated cells, with comparatively large and granular nuclei, are uniformly scattered through a homogeneous intercellular substance ; at the points of ossifica- tion the cells begin to assume a linear series, running down toward the ossifying surface, and separated from one another by the intercellular substance ; the cells are closely applied to one another, and so compressed that even their nu- clei seem often to touch ; the lowest rows rest in deep, narrow cups of bone, formed by the ossification of the intercellular substance ; the cups are gradually converted into closed areolce of bone, with their lamelliform walls. During this first stage of the process there are no blood vessels directly concerned. The lamellae of the areolas, or cancelli, become thicker, and include in their substance elongated oval spaces of a roughly granular nature, in other respects re- sembling lacunae, and considered by these ob- servers as the nuclei of the cells of the tem- porary cartilage; within the cancelli only a few cells are found, these cavities being chiefly occupied by a new granular substance, resem- bling a formative blastema, like that out of which all the tissues are evolved ; the cells are in apposition with the wall, and sometimes one seems half ossified, and its nucleus about to be- come a lacuna ; these nuclei have now the same direction as the neighboring lacunaa ; from the blastema the vessels are probably developed and the necessary elements for the growth of the bone. The cancelli, at first closed cavities, communicate at a subsequent period, and go to form the Haversian systems, a network of ves- sels becoming developed within them at the same time. The subsequent process of ossifica- tion consists essentially in the slow repetition of the above on the entire vascular surface of the bone. The canaliculi begin as irregularities in the margin of the lacunae, and are converted as the tissue becomes consolidated into the branching tubes which have been described above, and are accordingly formed in the ossi- fied substance of the cartilage cells. As to the lacunse, their granular interior seems to be gradually removed, and they become vacuities for the conveyance of the nutrient fluids. Agreeably to this theory of the formation of bone, Todd and Bowman believe that it grows chiefly by layers formed in succession on its vascular surface, but also in an interstitial manner after being originally deposited. A most important process of growth is constantly going on in cartilage by the multiplication of the cells and the increase in their dimensions ; in the long bones this growth is most active in the longitudinal direction. Bones also in- crease by the addition of new systems of lami- ( nte on their exterior, and by new involutions of the vascular surface to form new Haversian canals, as has been proved by experiments with madder mixed with the food of animals; the coloring principle of this substance has a re- markable affinity for phosphate of lime, and it affects first the portions of bone in course of formation, or those nearest to the vascular sur- face. "Wherever there is a vascular network in the structure of bone, whether on the peri- osteal or internal surface, there growth takes place ; the exterior increase is strictly analo- gous to the exogenous mode of growth in plants. A third mode in which bone grows seems to be by the dilatation of the primary cancelli and central Haversian canals ; by this enlargement