64 SIXTH LECTURE. 



When the bone is carefully decalcified its texture remains 

 as of old. It is easy to cut the mass, which has now become 

 semi-transparent. It is badly enough named the bone car- 

 tilage. 



In the mechanical construction of the body, the bones 

 come into consideration by reason of their solidity. They 

 serve as a protection to softer organs, and form systems of 

 levers moved by muscles. The less the proportion of bone 

 earths contained, the greater the flexibility and cohesion. A 

 preponderance of these mineral substances, on the contrary, 

 renders the bone inflexible and brittle. The mutability of its 

 materials is very considerable. In harmony with this is the 

 double system of canals for the blood-vessels and lacunae. 



The larger cavities of the bone become filled with so-called 

 bone marrow. This occurs in a double form, though with 

 transitions. In the central portion of the long bones it appears 

 as a yellow marrow, that is, as fat cells contained in loose 

 connective tissue (p. 50). In the epiphyses, on the contrary, 

 as well as in flat and short bones, we find a softer reddish or 

 red substance containing, together with scanty connective 

 tissue and isolated fat cells, very numerous lymphoid cells of 

 O.009 to 0.0113 mm. The latter elements, according to Neu- 

 mann and Bizzozero (p. 25), present transitions into red 

 blood corpuscles. Finally, we meet in the bone marrow, 

 especially towards the surface, the myeloplaxes, which have 

 become familiar to us in Fig. 13. The veins are without 

 endothelium ; they consist only of an adventitia (Hoyer). 

 Altogether, the vessels of the medulla promise still further 

 interesting conclusions. 



We now turn to the theory of the origin of the bone tissue, 

 osteogenesis. It forms a very difficult and complicated sec- 

 tion of histology. 



With the exception of a number of the bones of the cranium 

 and face, as we have already said, all portions of the skeleton 

 are preformed in cartilage. They afterwards present bone 

 substance. 



During a long period the direct metamorphosis of the 



