168 
HISTOLOGY OF ANIMALS. 
from the nuclei of the cartilage cells, but are cavities 
left in the newly formed bone, from which canaliculi are 
subsequently developed. The last described specimen 
of enchondroma, however, tends to prove that the view 
entertained by Todd and Bowman is the more correct. 
In order to understand what I am now about to 
describe, it will be necessary for me to give a brief 
account of the structure of bone. If we take, for 
example, one of the long bones of the body, we shall 
find that it consists of a shaft and two extremities ; if 
the same bone be divided transversely, its centre will 
be found to be hollow, or occupied by a spongy kind 
of bone, which has received the name of cancellated 
structure. Within the hollow, the marrow is contained, 
and the cavity, as you are well aware, is called from this 
circumstance the medullary cavity. If we examine the 
outer surface, we shall find a series of minute holes or 
foramina through which blood-vessels pass towards the 
interior of the bone. If now, 
a slice sufficiently thin to be 
transparent, be made and ex- 
amined with a power of forty 
diameters, a series of holes 
will be noticed, around which 
innumerable black spots are 
deposited in concentric cir- 
cles, as shown in Fig. 128 , 
these are the Haversian ca- 
nals, and through them the 
FIG. 128. 
Portion of a transverse section 
of a human femur magnified 40 
diameters. 
