THE BONE-CELLS. 



89 



tion to the blood-vessels, an extension of the marrow-tissue. The individual chan- 

 nels are short, and communicate by oblique branches with adjacent canals (Fig. 

 115). The Haversian canals indirectly communicate with the external surface of 

 the bone by means of the channels, or Volkmanii' s canals, within the circumfer- 

 ential lamellae, which open by minute orifices and receive vascular twigs from the 

 periosteal blood-vessels (Fig. 122); the latter are thus brought into free anasto- 

 mosis with the branches derived from the medullary vessels, the two constituting a 

 freely communicating vascular net-work throughout the compact substance. 



The Bone-Cells. — The details of 

 osseous tissue thus far considered per- Fig. 118. 



tain to the structure, of the passive in- 

 tercellular constituents of a dense con- 

 nective tissue ; in addition to these, as 

 in other forms of connective substances, 

 the more active elements are the con- 

 nective-tissue cells, here known as the 

 bone-cells. As already pointed out, the 

 lacunae and the canaliculi represent in- 

 tercommunicating lymph-spaces, similar 

 to those encountered in the cornea or 

 other dense connective tissue ; as in the 

 latter so also in the osseous tissue, the 

 cellular elements occupy the lymph- 

 spaces, the bone-cells lying within the lacunae. Since the classic pictures of bone 

 are derived from ground sections of dried tissue, in such preparations the deli- 

 cate bone-cells have shrunken and disappeared, and the lacunae contain, at best, 

 only the indistinguishable remains of the cells mingled with debris produced during 

 the preparation of the section ; the lacunae and the canaliculi in dried sections 

 are filled with air, by reason of which condition they appear as the familiar dark, 



sharply defined, conspicuous spider- 



Lacunae and canaliculi from dried bone cut parallel with 

 the lamellae. X 300. 



Fig. 119. 



 ■T^-"-ri"''"*~T"'7?i'-i'ffm7)rT"iMjliflii|i *"- 1 



like figures. 



In order to study the bone- 

 cells, the tissue after fixation is de- 

 calcified and stained, and mounted 

 in an approved preserving medium ; 

 in consequence of such treatment 

 the air is displaced from the spaces 

 within the bone, which now appear 

 faintly outlined, the delicate ramifi- 

 cations of the canaliculi in places 

 being almost invisible. The bone- 

 cells, after being stained in such 

 decalcified preparations, appear as 

 small lenticular or stellate bodies 

 within the lacunae (Fig. 121), which 

 they almost entirely fill. Each cell- 

 body consists of granular cytoplasm 

 from which delicate processes ex- 

 tend for a variable distance into the 

 canaliculi, in favorable localities the 

 protoplasmic processes sent out by adjacent bone-cells sometimes meeting. The 

 deeply staining nucleus appears as a brilliant point within the stellate cell. 



The Periosteum. — The external surface of bones is closely invested, except 

 where covered with cartilage, with a fibrous membrane, the periosteum, a structure 

 of great importance during development and growth, and later for the nutrition 

 and protection of the osseous tissue. During childhood an end of the immature 

 bone may be broken off and yet held in place by the periosteum. The adult peri- 

 osteum consists of two layers, an outer fibrous and an inner fibro-elastic ; when 

 covering young bones, however, in which growth is actively progressing, the peri- 



v 



Semi-diagrammatic view of perforating fibres of Sharpey ; the 

 iamellae of decalcified bone have been partially separated. 



