BONE 79 



of the bone and marrow and form a rich perivascular plexus which is 

 distributed to the walls of the vessels; occasional side fibrils are also dis- 

 tributed to the marrow. Nerve endings have not been demonstrated in 

 compact bone nor in the articular cartilages. In the periosteum terminal 

 nerve fibrils are supplied to the musculature of the blood-vessels, and 

 other sensory fibrils end in lamellar corpuscles. 



Development of Bone Bone tissue makes its appearance relatively 

 late ill fetal life. The long bones are first mapped out by masses of 

 hyaline cartilage. The entire skeleton, with the exception of the flat 

 bones of the face and those of (lie vault of the skull, is thus primarily 

 formed by plates of fetal cartilage. The process by which these cartilag- 

 inous plates are transformed into bone is known as intracartilaginous or 

 enclwndral ossification. The process is essentially one of replacement 

 of cartilage by bone, not one of change of cartilage into bone. The 

 resulting bones are known as substitution bones. 



The flat bones of the face and skull (including the interparietals, 

 parietals, frontals, squamosals, tympanics, median pterygoid plate of the 

 sphenoid, nasals, lacrimals, malars, palatine, vomer, maxilla, and a 

 portion of the mandible) are formed directly from the mesenchymal 

 blastema without the intervention of cartilage. This method of bone 

 formation differs somewhat from the above and is known as intramem- 

 b ranous ossificatio n . 



INTRACARTILAGINOUS OSSIFICATION. This process begins with the 

 formation of plates of hyaline cartilage whose shape corresponds more or 

 less closely with that of the future bone. This type of fetal cartilage 

 differs from the hyaline cartilage of the adult only in the irregular form 

 and distribution and greater abundance of its cartilage cells. 



Each plate of fetal cartilage is enveloped by a layer of embryonal 

 fibrous tissue, the fetal perichondrium. The outer portion of the fibro- 

 cellular layer is destined to become the periosteum of the future bone; 

 its innermost portion contains many small round cells, which from their 

 intimate relation to bone production, are known as osteoblasts. The 

 inner portion of the perichondrium forms the osteogenic layer of the 

 future periosteum. 



Centers of Ossification. Ossification of the cartilage begins at one or 

 more points which are called centers of ossification. In the long bones, 

 in which the process of bone formation can be most readily traced, there 

 are usually three such centers, one near the middle of the cartilaginous 

 plate, from which the diaphysis is formed, and one epiphysial center at 

 each extremity. The centers for the epiphyses make their appearance 



