

BONE 95 





primary spicules goes on pari passu with the formation of permanent bone by 

 the periosteum, and in this way the medullary cavity of the body of the bone is 

 formed. 



This series of changes has been gradually proceeding toward the end of the body 

 of the bone, so that in the ossifying bone all the changes described above may 

 be seen in different parts, from the true bone at the center of the body to the hyaline 

 cartilage at the extremities. 



While the ossification of the cartilaginous body is extending toward the articular 

 ends, the cartilage immediately in advance of the osseous tissue continues to grow 

 until the length of the adult bone is reached. 



During the period of growth the articular end, or epiphysis, remains for some 

 time entirely cartilaginous, then a bony center appears, and initiates in it the 

 process of intracartilaginous ossification; but this process never extends to any 

 great distance. The epiphysis remains separated from the body by a narrow 

 cartilaginous layer for a definite time. This layer ultimately ossifies, the distinc- 

 tion between body and epiphysis is obliterated, and the bone assumes its completed 

 form and shape. The same remarks also apply to such processes of bone as are 

 separately ossified, e. g., the trochanters of the femur. The bones therefore con- 

 tinue to grow until the body has acquired its full stature. They increase in length 

 by ossification continuing to extend behind the epiphysial cartilage, which goes 

 on growing in advance of the ossifying process. They increase in circumference 

 by deposition of new bone, from the deeper layer of the periosteum, on their exter- 

 nal surface, and at the same time an absorption takes place from within, by which 

 the medullary cavities are increased. 



The permanent bone formed by the periosteum w r hen first laid down is cancellous 

 in structure. Later the osteoblasts contained in its spaces become arranged in 

 the concentric layers characteristic of the Haversian systems, and are included 

 as bone corpuscles. 



The number of ossific centers varies in different bones. In most of the short 

 bones ossification commences at a single point near the center, and proceeds toward 

 the surface. In the long bones there is a central point of ossification for the body 

 or diaphysis: and one or more for each extremity, the epiphysis. That for the 

 body is the first to appear. The times of union of the epiphyses with the body 

 vary inversely with the dates at which their ossifications began (with the exception 

 of the fibula) and regulate the direction of the nutrient arteries of the bones. Thus, 

 the nutrient arteries of the bones of the arm and forearm are directed toward 

 the elbow, since the epiphyses at this joint become united to the bodies before 

 those at the opposite extremities. In the lower limb, on the other hand, the 

 nutrient arteries are directed away from the knee: that is, upward in the femur, 

 ownward in the tibia and fibula; and in them it is observed that the upper epiphysis 

 f the femur, and the lo\ver epiphyses of the tibia and fibula, unite first with the 

 odies. Where there is only one epiphysis, the nutrient artery is directed tow r ard 

 the other end of the bone; as toward the acromial end of the clavicle, toward the 

 distal ends of the metacarpal bone of the thumb and the metatarsal bone of the 

 great toe, and toward the proximal ends of the other metacarpal and metatarsal 

 bones. 



Parsons 1 groups epiphyses under three headings, viz.: (1) pressure epiphyses, 

 appearing at the articular ends of the bones and transmitting "the weight of the 

 body from bone to bone;" (2) traction epiphyses, associated with the insertion 

 of muscles and "originally sesamoid structures though not necessarily sesamoid 

 bones;" and (3) atavistic epiphyses, representing parts of the skeleton, which at 

 one time formed separate bones, but which have lost their function, "and only 

 appear as separate ossifications in early life." 



1 Jour, of Anat. and Phys., vols. xxxviii, xxxix, and xlii. 





