LECTURE IV. 



this concentric arrangement around the vessels con- 

 stantly prevails. 



Between these more lamellated parts is left a small 

 quantity of osseous substance (Fig. 33, i) which does not 

 present the same structure, but is arranged upon another, 

 and independent, plan. Upon more accurate examina- 

 tion it is seen to be formed of little columns, which are 

 generally perpendicular to the long axis of the bone, but 

 sometimes curve round, and so become parallel to the 

 long axis. These are the remains of the spicula first 

 formed during the growth of the bone in thickness, and 

 are therefore of older date. 



As in the sections which are obtained by grinding 

 down bone, the vessels themselves cannot for the most 

 part any longer be distinguished, the cavities [Haversian 



FIG. 34. 



canals] (Fig. 32, a, 33, a, 0,) in which they run have 

 been named medullary canals, improperly, inasmuch as 



Fig. 34. Bone-corpuscles from a morbid formation of bone in the dura mater of 

 the brain. Their branching and anastomosing prolongations (canaliculi) are seen, 

 as well us minute spots upon their walls, marking the funnel-shaped commence- 

 ments of the canaliculi. 600 diameters. 



