SYSTEMATIC SYNTHESIS OF TISSUES. 



187 



proportions from the first till the close of life, producing 

 great brittleness of the bones in advanced years. 



FIG. 149. 



Fjg. 149 represents, at 3, a small portion taken from the centre of a 

 bone, having a thick, dense exterior, and a central canal ; at 1 the dense 

 part growing thinner, enclosing 2, a mass of curiously wrought bone, look- 

 ing like very fine sponge, and called cancellated. The appearance can 

 be seen in almost any bone. 



FIG. 150. FIG. 151. 



Fig. 150 represents a magnified portion of bone, and the arrangement 

 of matter around the canals, a, b, forming hollow rods ; c, tissue packed 

 between the cylinders. The canals are channels for the location of blood- 

 tubes, nerves, etc., for nourishing the bone. 



Fig. 151, a, represents, in black, one canal, ff, c, and very much enlarged 

 lacunae, arranged around it ; b, an irregular cluster between cylinders. 

 The branches from the lacunae are seen to communicate in some cases. 

 Thus nourishment has the opportunity of reaching every part of a bone. 



Describe Fig. 149. Describe Fig. 150. Describe Fig. 151. When will bones 

 most readily be broken ? 



