58 



GENERAL ANATOMY. 



FIG. 28. Nucleated bone-cells and their 

 processes, contained in the bone-lacunee and 

 their caualiculi respectively. From a section 

 through the vertebra of an adult mouse. 

 '(Klein and Noble Smith.) 



The lacunae are situated between the lamellae, and consist of a number of 

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

 they appear as dark, oblong, opaque spots, and were formerly believed to be solid 

 cells. Subsequently, when it was seen that the Haversian canals were channels 

 which lodge the vessels of the part, and the canaliculi minute tubes by which the 

 plasma of the blood circulates through the tissue, it was taught that the lacunae 

 were hollow spaces filled during life with the same fluid, and only lined (if lined 

 At all) by a delicate membrane. But this view appears also to be delusive. 



Examination of the structure of bone, when 

 recent, led Virchow to believe that the lacunae 

 are occupied during life with a nucleated cell, 

 the processes from which pass down the canal- 

 iculi a view which is now universally received 

 (Fig. 28). It is by means of these cells that 

 the fluids necessary for nutrition are brought 

 into contact with the ultimate tissue of bone. 



The canaliculi are exceedingly minute 

 channels, which pass across the lamellae and 

 connect the lacunae with neighboring lacunae 

 and also with the Haversian canal. From this 

 central canal a number of the canaliculi are 

 given off, which radiate from it, and open into 

 the first set of lacunae, arranged around the 

 Haversian canal, between the first and second 

 lamellae. From these lacunae a second set of 

 canaliculi are given off, which pass outward to 

 the next series of lacunae, and so on until they reach the periphery of the Haver- 

 sian system ; here the canaliculi given off from the last series of lacunae do not 

 communicate with the lacunae of neighboring Haversian systems, but after passing 

 outward for a short distance form loops and return to their own lacuna. Thus 

 every part of an Haversian system is supplied with nutrient fluids derived 

 from the vessels in the Haversian canals and traversing the canaliculi and 

 lacunae. 



The bone-cells are contained in the lacunae, which, however, they do not com- 

 pletely fill. They are flattened nucleated cells, which Virchow has shown are 

 homologous with those of connective tissue. The cells are branched, and the 

 tranches, especially in young bones, pass into the canaliculi from the lacunae. 



If a longitudinal section is examined, as in Fig. 29, the structure is seen to be 

 the same. The appearance of concentric rings is replaced by that of lamellae or 

 rows of lacunae, parallel to the course of the Haversian canals, and these canals 

 appear like half-tubes instead of circular spaces. The tubes are seen to branch 

 and communicate, so that each separate Haversian canal runs only a short distance. 

 In other respects the structure has much the same appearance as in transverse 

 sections. 



In sections of thin plates of bone (as in the walls of the cells which form the 

 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 func- 

 tion as the Haversian canals in the more compact bone. 



Chemical Composition. Bone consists of an animal and an earthy part inti- 

 mately combined together. 



The animal part may be obtained by immersing the 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. 

 30), the same general arrangement of the Haversian canals, lamellae, lacunae, and 

 canaliculi is seen, though not so plainly, as in the ordinary section. 



