dded in bony matrix in a little bed, the
lacuna, and communicating one with another by fine processes
through canaliculi in the matrix, which processes are only to be
seen clearly in decalcified bone (See Section 70). The osteoblasts are
arranged in concentric series, and the matrix is therefore in concentric
layers, or lamellae (c.l.). Without and within the zone of Haversian
systems are (o.l. and i.l.), the outer and inner lamellae. The bone is
surrounded by connective tissue, the periosteum. In addition to this
compact bone, there is a lighter and looser variety in which spicules
and bars of bony tissue are loosely interwoven. Many flat bones, the
bones of the skull, for instance, consist of this spongy bone, plated
(as an electro spoon is plated) with compact bone.

Section 69. Among the bony bars and spicules of spongy bone
occurs the red marrow-- which must not be confused with the yellow
marrow, the fatty substance in the central cavity of long bones. In this
red marrow are numerous large colourless cells, which appear
to form within their substance and then liberate red blood corpuscles.
This occurs especially in the spongy bone within the ribs.

Section 70. The matrix of bone differs from that of cartilage or of
most other tissues in consisting chiefly of inorganic salts. The chief of
these is calcium phosphate, with which much smaller quantities
of calcium carbonate, and magnesium phosphate and carbonate
occur. These inorganic salts can be removed by immersion of the
bone in weak hydrochloric acid, and a flexible network of connecting
tissue, Haversian vessels, bone corpuscles, and their processes
remains. This is decalcified bone alluded to above.

Section 71. In the very young rabbit, the limb bones, vertebral column,
and many of the skull bones are simply plates and bars of cartilage;
the future membrane bones, however are planned out in connective
tissue. The development of the latter is simple, the connective
tissue corpuscles functioning by a simple change of product as
osteoblast. The development of the cartilage bones, however, is more
complicated. Figure XVII., represents, in a diagrammatic way, the
stages in the conversion of a cartilaginous bar to bone. To begin with,
the previously sporadically-arranged (scattered anyhow) corpuscles
(u.c.c.) are gathered into groups in single file, or in other words, into
"columnar" groups (as at c.c.). The matrix becomes clouded with
inorganic salts of li