OSSIFICATION, or the formation of bone, is a process to which physiologists have paid much attention, but regarding which there is still considerable difference of ion. On one point, however, there is a general agreement—viz., that the bones are not in any instance a primary formation, but always result from the transfor•ition and earthy' impregnation of some pre-existing tissue, which is most commonly either carti lage or a membrane containing cell-nuclei.. At a very early period of embryonic life, as soon, indeed, as any structural differences can be detected, the material from which the bones are to lie formed becomes mapped out as soft gelatinous substance, which m iy be distinguished from the other tissues by being rather less transparent, and soon beco n ing decidedly opaque. From this beginning the bones are formed in two ways; either the tissue just described becomes converted into cartilage, which is afterwards repl•ced by bone, or a gertninal membrane is formed, in which the ossifying process takes place. The latter is the most simple and rapid mode of forming bone. When ossification co n mences, the membrane becomes more opaque, and exhibits a decided fibrous character, the fibers being arranged more or less in a reticulated manner. These fibers become more distinct and granular from impregnation with lime salts, and are converted in incipient bone, while the cells which are scattered among them shoot out into the bone corpuscles, from which the canaliculi are extended probably by resorption.., The facial and cranial bones, with the exception of those at the base of the skull, are thus formed without the intervention of any cartilage.
The process of ossification in cartilage (q.v.) is too complex and difficult to follow in these pages. Some physiologists hold that when ossification is carried on in cartilage a complete molecular replacement of one substance by the other takes place; while others believe that more or less of the cartilaginous matrix remains, and becomes impregnated with earthy matter at the same time that gluten is substituted for chondrine (chondrine being the variety of gelatine that is yielded by ossein or bone-cartilage before ossification, while gluten is yielded after that process is established). All the bones of the body, excepting those of the head and face already mentioned, are at first formed, in part at all events, from cartilage.
The time at which ossification commences does not at all follow the order in which the primordial cartilage is laid down. Thus the cartilage of the vertebra appears before there is any trace of that of the clavicle; yet at. birth the ossification of the latter is
almost complete, while that of the former is very imperfect for many years. We will briefly trace the process of ossification as it occurs in the human femur or thigh-bone. Ossification commences in the interior of the cartilage at determinate points, which are hence termed points or centers of ossification. From these points the process advances into the surrounding substance. In the second month of foetal life,.one of these centers shows itself about the middle of the shaft, and from this point ossification rapidly extends upwards and downwards along the whole length of the shaft. The upper and lower ends remain cartilaginous, and it is not till the last month of foetal life that a second cen ter. appears at the lower end. The third center, from'which the upper end of the bone is ossified, does not appear till about a year after birth. The bone now consists of two extremities, or epiphyses, with an intermediate shaft or diaphysis; and the superior epip hysis is not ossified to the shaft until about the 18th, and the inferior until after the 20th.
year. At about the 5th year a fourth ossific center is developed in the cartilage of the• greater trochauter, and a fifth center appears in the lesser trochanter at about the 14th year. These osseous processes, thus developed from special ossific centers, are termed apophyses. Most of the long bones are developed in a corresponding way. It is a curi ous fact (which is of such general occurrence that it may be regarded as a law) that in the skeletons, both of man and of the lower animals, the union of the various apophyses to the epiphyses, and of the epiphyses to the diaphysis or shaft, takes place in the inverse order to that in which their ossification began. The advantages derived from this sub division of the long bones into segments, with interposed cartilaginous plates, are obvi ous. Besides the greater facilities for growth thus afforded, the flexibility of the bony frame-work is thereby greatly increased, and its escape from injury during the many falls incick.ntal to this period of life is in no small degree attributable to this cause. See Humphry On the Human Skeleton, pp. 33 Lo 45. ' True ossification sometimes occurs as a morbid process; but in many cases, the term is incorrectly used (especially in the case of blood-vessels) to designate a hard calcareous deposit, in which the characteristic microscopic appearances of true bone are altogether absent.