CARTILAGE. BONE. TEETH. 81 



medium in which they are found, which we shall 

 designate as the fundamental substance of hone. This 

 latter consists of a whitish structureless material 

 opaque, or transparent, according to the thickness of 

 the section. It is composed, chemically, of earthy 

 salts, and an organic substance by means of which the 

 earthy particles are held together. 



The cells of bone (called also osseous corpuscles, 

 osteo-plastic cells, and laounce) bear some resemblance 

 in their shape and outline to the star-shaped or 

 branching plasmatic cells already described. They 

 are minute fusiform bodies, slightly flattened laterally, 

 and measuring from T J 7 th to T ~d of a line in length. 

 From their exterior a delicate tracery of minute 

 thread-like prolongations radiate in every direction, 

 anastomosing with each other, and with those of neigh- 

 boring cells. Under a magnifying power of from 350 

 to 400 diameters it can be distinctly seen that these 

 filiform appendages of the cells of bone are hollow in 

 their interior, and are, in fact, very minute tubes or 

 canaliculi ; their mode of communication is likewise 

 very apparent, (PL V. fig. IV). The more delicate 

 scales of the spongy variety of bone, and the cemen- 

 tum of the teeth, present in fact no other constituent 

 elements ; but this is not true of the more dense or 

 cortical substance of bone, or of scales of greater thick- 

 ness. On placing a transverse section of a long bone 

 under the microscope, it is at once apparent that its 

 cells are grouped after a certain fixed plan. In fact 

 they are arranged very regularly in concentric circles 

 around a larger central opening which is the trans- 

 verse section of the track of a blood-vessel, or in other 



