74 NORMAL HISTOLOGY. 



In the spongy or cancellated variety of hone the thin plates of 

 that tissue derive their nourishment from the lymph of the con- 

 tiguous marrow rilling the spaces between them, and there is no 

 occasion for Haversian canals. The concentric arrangement of the 

 laminae is, therefore, absent. 



Except where bounded by cartilage at the joints, the external 

 surfaces of the bones are covered by a fibrous investment, the 

 periosteum, in which the bloodvessels supplying the bone ramify 

 and subdivide before sending their small twigs into the Haversian 

 canals of the compact bone. A few nerve-fibres also penetrate into 

 the bone from the periosteum. The deep surface of the periosteum 

 contains connective-tissue cells, " osteoblasts," capable of assuming 

 the functions of bone-corpuscles and producing bone. In places 

 where the bone is developing, these osteoblasts are large and cyto- 

 plasmic, somewhat resembling the cells of cuboidal epithelium. 

 These facts explain the importance of the periosteum for the nutrition 

 and growth of bone. The tendons and ligaments attached to the 

 bones merge with the periosteum, which has a similar fibrous struct- 

 ure and serves to connect them firmly with the surface of the 

 bone. 



The central cavities of the long bones and the spaces of cancel- 

 lated bone are occupied by marrow, which may be of two kinds, the 

 " red " or the " yellow." A description of the structure of marrow 

 must be deferred until the other varieties of the connective tissues 

 have been considered. 



In the embryo the parts which are destined to become bony first 

 consist of some other variety of connective tissue, either cartilage 

 or fibrous tissue. This subsequently "ossifies," during which proc- 

 ess it is not really converted into bone, but is gradually absorbed 

 as that tissue develops and replaces it. 



Because of its hardness and compact structure it is, perhaps, a 

 little difficult to realize the plastic character of bone, and to under- 

 stand the growth and gradual changes that take place in a tissue that 

 would seem to be so permanent. These changes are wrought by the 

 absorption of bone already formed and the deposition of new bone. 

 The long bones increase in length as long as the epiphyseal cartilage 

 persists; this cartilage grows and is constantly being replaced by 

 bone formed at its junction with the shaft. Bones increase in diame- 

 ter by the deposition of fresh osseous tissue produced by the osteo- 

 blasts forming the deep layer of the periosteum. As the osteoblasts 



