50 THE MICROSCOPE. 
With a yet higher power examine the lacune more closely. 
Many of them will be seen to contain a nucleated cell, conforming 
in shape with the lacunze and sending delicate processes into the 
canaliculi. 
Tre Haverstan Canars.—Longitudinal sections will show that 
the course taken by these canals is in the direction of the long axis 
of the bone and that they anastomose freely with each other and com- 
municate eventually with the central canal. They are lined with a 
delicate, structureless membrane and contain one or two small blood- 
capillaries, a nerve filament and a lymphatic vessel. The remaining 
space, if any, contains a few marrow cells, or occasionally, in the large 
canals, small quantities of delicate fibrils. 
THe Marrow.—Marrow is of two sorts, red and yellow. Red 
marrow is found in the cancellated bone and the softer, spongy por- 
tions of compact bones. 
From a fresh bone extract a small portion of marrow, stain in 
carmine or picro-carmine and examine in glycerin. The yellow 
marrow will be seen to be composed principally of adipose tissue. 
Together with this are a number of round, nucleated, lymphoid-like 
cells, the marrow. cells. Like the lymphoid cells, they possess the 
power of amceboid movement. 
Red marrow is much more vascular and contains less fatty 
tissue. There are also a number of reddish-green, nucleated, amce- 
boid cells of different shapes and sizes, which are supposed to be cells 
undergoing transition from white to red blood-corpuscles. Lastly, 
are a number of very large, multi-nucleated giant cells (mycloplaxes 
of Robin) found also in the white marrow. 
CanceLLaTeD Bonze.—Though differing in some details from 
compact bone it contains all the elements which go to make up an 
Haversian system in the latter form. The chief difference is to be 
found in the marrow spaces. Aside from their containing a red marrow 
they are unlike the Haversian and central canals of compact bone, 
in that they are much larger, more irregular and are not distinctly 
tubular. If it were possible to inflate a long, compact bone through 
its central canal, and thus cause the Haversian canals to expand and 
bulge out irregularly, a picture somewhat resembling a cancellated 
bone might be produced. 
Tue Periostrum.—This fibro-elastic membrane which covers the 
bone can best be studied in sections made from decalcified bone, 
although it may be necessary to examine a number of sections 
to find a clear example. It is composed of three layers: (1) 
