STRUCTURE OF BONE. 9 
lamelle are arranged cither in a concentric manner 
around a blood-vessel (Haversian canal), or as inter- 
stitial or ground Jamelle. 
Each Haversian canal contains two blood-vessels, 
an artery and vein, surrounded by connective tissue. 
The canals pass in the bone in a longitudinal direction, 
giving off transverse or oblique branches ; and so anas- 
tomose with one another. They open on to the osteo- 
genetic layer of the periosteum by fine pores, and into 
the medullary cavity; as the canals approach the 
medullary cavity, they gradually enlarge, and finally 
fuse with the marrow tissue. The concentric lamellz 
surround the Haversian canals, while the interstitial 
or ground lamelle fill in the space between the 
Haversian systems. 
In the spongy portion of the bone, the cavities 
are termed Haversian spaces, and are usually filled with 
a red marrow; the bone substance here forming a 
series of septa or lamella, which are termed the bone 
trabecule. 
The centre or hollow cavity of a bone is termed 
the medullary cavity; surrounding this internal cavity 
in the form of a lining membrane is a vascular layer 
of areolar tissue the endosteum. 
The marrow is a fibro-vascular tissue, consisting 
of a matrix of retiform and adipose tissue, blood- 
vessels and cells. The marrow cells are termed yellow 
marrow and red marrow respectively. The former 
