

BONE 



91 



spaces. In an ordinary microscopic section, viewed by transmitted light, they 

 appear as fusiform opaque spots. Each lacuna is occupied during life by a branched 

 cell, termed a bone-cell or bone-corpuscle, the processes from which extend into the 

 canaliculi (Fig. 76). 



The Canaliculi are exceedingly minute channels, crossing the lamellae and con- 

 necting the lacunae with neighboring lacunae and also with the Haversian canal. 

 From the Haversian canal a number of canaliculi are given off, which radiate from 

 it, and open into the first set of lacunae between the first and second lamellae. 

 From these lacunae a second set of canaliculi is given off; these run outward to the 

 next series of lacunae, and so on until the periphery of the Haversian system is 

 reached; here the canaliculi given off from the last series of lacunae do not communi- 

 cate with the lacunae of neighboring Haversian systems, but after passing outward 

 for a short distance form loops and return to their own lacunae. Thus every 

 part of an Haversian system is supplied with nutrient fluids derived from the 

 vessels in the Haversian canal and distributed 

 through the canaliculi and lacunae. 



The bone cells are contained in the lacunae, 

 which, however, they do not completely fill. 

 They are flattened nucleated branched cells, 

 homologous with those of connective tissue; the 

 branches, especially in young bones, pass into 

 the canaliculi from the lacunae. 



In thin plates of bone (as in the walls of 

 the spaces of cancellous tissue) the Haversian 

 canals are absent, and the canaliculi open into 

 the spaces of the cancellous tissue (medullary 

 spaces), which thus have the same function as 

 the Haversian canals. 



Chemical Composition. Bone consists of an 

 animal and an earthy part intimately com- 

 bined together. 



The animal part may be obtained by immersing a bone for a considerable time 

 in dilute mineral acid, after which process the bone comes out exactly the same 

 shape as before, but perfectly flexible, so that a long bone (one of the ribs, for 

 example) can easily be tied in a knot. If now a transverse section is made 

 (Fig. 77) the same general arrangement of the Haversian canals, lamellae, lacunae, 

 and canaliculi is seen. 



The earthy part may be separately obtained by calcination, by which the 

 animal matter is completely burnt out. The bone will still retain its original 

 form, but it will be white and brittle, will have lost about one-third of its original 

 weight, and will crumble down with the slightest force. The earthy matter is 

 composed chiefly of calcium phosphate, about 58 per cent, of the weight of the 

 bone, calcium carbonate about 7 per cent., calcium fluoride and magnesium phos- 

 phate from 1 to 2 per cent, each and sodium chloride less than 1 per cent. ; they confer 

 on bone its hardness and rigidity, while the animal matter (osseiri) determines its 

 tenacity. 



Ossification. Some bones are preceded by membrane, such as those forming 

 the roof and sides of the skull; others, such as the bones of the limbs, are preceded 

 by rods of cartilage. Hence tw r o kinds of ossification are described: the intra- 

 membranous and the intracartilaginous. 



INTKAMEMBRANOUS OSSIFICATION. In the case of bones which are developed 

 in membrane, no cartilaginous mould precedes the appearance of the bony tissue. 

 The membrane which occupies the place of the future bone is of the nature of con- 

 nective tissue, and ultimately forms the periosteum; it is composed of fibers and 

 granular cells in a matrix. The peripheral portion is more fibrous, while, in the 



FIG. 76. Nucleated bone cells and their 

 processes, contained in the bone lacuna and 

 their canaliculi respectively. From a section 

 through the vertebra of an adult mouse. 

 (Klein and Noble Smith.) 



