The Bulletin 
5 
are composed of fine white fibrous tissue fibrils imbedded in a ground 
substance in which they are arranged in layers superimposed in such 
a way that the fibers in the several layers cross at right angles, forming 
an angle of 45 degrees with a long axis of haversian canals. 
In each lacuna there is found a nucleated hone cell which practically 
fills it, with its processes extended out into the canaliculi. 
The haversian canals contain blood vessels, either an artery or vein 
or both. Between the vessels and the walls of the canals are perivascular 
spaces bounded by endothelial cells. Into these spaces the canaliculi 
of the haversian system empty. All lacunae and canaliculi should be 
considered as filled by lymph plasma which circulates throughout the 
bones, bathing the bone cells and the processes. 3 In the fetal skeleton 
it is found that some bones are formed from a fibro-elastic membrane, 
such as those forming the roof and side of the skull. Others, such as 
the bones of the extremities, are formed from cartilage. Thus we 
find two kinds of ossification, intrafibrous and intramembranous and 
intracartilaginous. 4 
In the intrafibrous ossification no cartilaginous mould or stage pre¬ 
cedes the appearance of the bone tissue. The membrane which occupies 
the place of the future bone is of the nature of connective tissue, and 
ultimately forms the periosteum. The outer portion is more fibrous 
than the internal, in which the cells or osteoblasts predominate. The 
whole tissue is richly supplied with blood vessels. 
Beginning of the Process 
At the onset of the process of bone formation a small network of bone 
spiculae is first noticed, radiating from the point or center of ossification. 
A miscroscopic study shows it to consist of a network of fine, clear 
fibers with an intervening ground substance. These are called osteo¬ 
genic fibers. They soon take on a dark granular appearance from the 
deposit of calcareous granules in the fibers and the intervening matrix. 
As they calcify they are found to inclose some of the granular corpuscles 
or osteoblasts. The latter form the corpuscles of the future bone, and 
the spaces in which they are inclosed are the lacunae. As the osteogenic 
cells grow to the periphery they calcify and then give rise to fresh bone 
spicuke. 
The meshes of the network of bone thus formed contain blood vessels 
and delicate connective tissue crowded with osteoblasts. These bone 
spicuke thicken by the accretion of layers formed by the osteoblasts, and 
the meshes become smaller. Subsequently separate layers of bone tissue 
are deposited in the periosteum and around the large vascular channels. 5 
In the intracartilaginous ossification we find the primary bone en¬ 
tirely cartilaginous, and in the long bones the process commences in the 
center (shaft ossification) and extends toward the extremities, which for 
