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BONES. 47 :

blast. These deposits, which on cross-section have a crescentic shape,
become thicker and thicker, rising- up around the cell, which they iinally
enclose—the enclosed osteoblast becoming, as it would seem, a bone-cell.
This process occurring around each osteoblast, the walls of the medullary
cavities soon become covered with a layer of bone containing bone-cells.
New osteoblasts appear on the walls, and in turn become enclosed in a
layer of bone, and thus the lamellar arrangement of bone-tissue is pro-
duced. The remains of cartilage basement-substance between the
medullary space thus covered by bone finally disappear in a manner
unknown to us." ^
II. The Subperiosteal Bone is the portion formed on the outer
surface of that which is developed within the cartilage, and by the
formation of which bones increase in thickness. It is deposited in a
manner similar to endochondral bone, through the influence of osteo-
blasts found on the inner portion of the osteogenetic layer of the peri-
chondrium, which has now become periosteum. The osteoblasts are
arranged along the line of blood-vessels and connective-tissue bundles
of the osteogenetic layer of the periosteum, and as these structures are
not parallel to each other or to the surface of the bone, but cross at
various angles, forming an uneven network, they cause newly-formed
bone to have an uneven surface, with branching grooves and canals
passing in diiferent directions. Upon the walls or sides of these grooves
and canals the osteoblasts, by means of which bone-tissue is deposited,
are distributed, spaces being left for blood-vessels and marrow-tissue:
these spaces subsequently become the Haversian canals. During
the time these canals and spaces are being encroached upon by ossific
deposit newer layers are commenced on the outer surface of the bone,
the fully-formed or ossified layer being continually overlaid by fresh
coatings in a manner similar to the lamellne of the Haversian system
by this process the bone grows in thickness. These lamellne are held
or bound together by perforating fibres (Sharpey's fibres), which pass
through several layers at nearly right angles ^^'ith the surface. These
fibres originate from the bundles of connective tissue of the subperios-
teal membrane, but do not all have connection with the periosteum
itself, though doubtless they had their origin from that membrane, the
same as the bone in which they are found. Perforating fibres are
usually ossified, but in some instances they are not, and in the drying
of the bone they become shrunken, leaving perforations.
In the long bones the caitilage grows and extends toward the
epiphyses, where, by gradually incr&asing in diameter, it causes the can-
cellated portion of the bone to ])resent a somewhat similar shape to that
of an elongated hour-glass. Where deposition of bone first commenced
the cancellated part is the narrowest, and the cortical portion, which grows
from the periosteum, is the thickest. Toward the ends of the bones
the cortical substance gradually diminishes to a thin layer, thus main-
taining a nearly equal diameter for the bone from end to end, A little
before or about the time of the development of the periosteal bone the
central portion of the embryonal spongy or endochondral bone under-
goes a process of softening or absorption (osteoporosis, Schwalbe). In
^ Pruden's Practical Histoiogy, 2d ed., p. 73.