5 o NORMAL HISTOLOGY. 



the connective tissue. The details of this bone-formation by the osteoblasts 

 correspond to those seen in other localities, including the conversion of 

 some of the osteoclasts into bone-cells. With the growth of the bony 

 tissue the network becomes more and more compact until it forms an osseous 

 plate, which gradually expands towards" the limits of the future bone. 

 During this growth, the connective tissue covering the outer and inner sur- 

 faces of the plate assumes the character and arrangement of a periosteum and 

 from the osteogenetic layer produces the compact surface lamellae which 

 enclose the intervening spongy tissue. This arrangement is seen in the fully 

 developed bone (Fig. 48), where the so-called outer and inner tables enclose 

 the diploe. The development of the superficial layers of the surface lamellae 

 is, therefore, identical with that of other subperiosteal surface lamellae, while 

 the production of the diploe corresponds with that of the peripheral spongy 

 bone in its essentials, even to partial absorption in order to produce enlarged 

 marrow spaces. The secondary deposit of Haversian lamellae, however, 

 never takes place, the conspicuous systems of concentric layers being absent 

 in the membrane-bones. Increased thickness of the membrane-bones follows 

 the addition of new surface lamellae ; increased area results from the marginal 

 growth of the enclosed network of bony trabeculae. In the young skull the 

 vault-bones are separated by considerable tracts of connective tissue, con- 

 spicuous in the fontanelles and the evident sutures. This isolation continues, 

 in principle at least, even after the bones are in close apposition, and ends 

 only with the complete replacement of the intervening periosteum by bone, 

 such bony union being subject to great individual variations as to time 

 and extent. 



Growth of Bones. Since new bone is deposited beneath the peri- 

 osteum, it is evident that in a long bone such growth results in increased 

 diameter of the shaft, as well as in increased thickness of the bony wall 

 between the central medullary cavity and the surface. In order to maintain 

 the balance between the longitudinal growth of the marrow-cavity (effected 

 by the destruction of the cartilage and the absorption of the intracartilagi- 

 nous bone) and its lateral expansion, removal of the innermost layers of 

 the subperiosteal bone soon becomes necessary. This is effected by the 

 osteoclasts, absorption of the older internal portion accompanying the depo- 

 sition of new lamellae on the surface. By this combination of destructive 

 and formative processes, the thickness of the cylindrical wall of compact 

 substance of the shaft is kept within the proper limits to insure the necessary 

 strength without undue weight. During early growth, increase in the length 

 of the bone is due to the addition at the ends of new cartilage formed by the 

 perichondrium ; later, these additions are supplemented by interstitial growth 

 following multiplication of the cartilage-cells. On attaining full growth and 

 completed epiphyseal ossification (page 47), a portion of the cartilage per- 

 sists as the covering of the articular surfaces. During the development of 

 the short bones, in which the entire bone is made up by a mass of spongy 

 substance enclosed by a shell of compact bone, no definite envelope of sub- 

 periosteal bone forms until the cartilage has completely disappeared. The 

 subsequent growth and expansion of such bones is accomplished by the 

 superficial addition and internal absorption of the subperiosteal bone and the 

 accompanying expansion of the central spongy tissue. In the flat bones, as 

 the scapula, the subperiosteal production is well advanced before the intra- 

 cartilaginous process begins. After the cessation of peripheral growth, the 

 osteogenetic layer of the periosteum becomes denser and much less rich in 

 cells, although it retains an intimate connection with the last formed lamellae 



