THE SKELETON IN GENERAL. 507 
In certain situations the lamellz separate, and leave between thein spaces 
of various sizes, called cancel. Besides entering into the composition of 
the basis substance, the lamellz are collected concentrically round the 
Haversian canals, the boundaries of which they form, generally to the 
extent of ten to fifteen layers. Both the compact and spongy tissues are, 
therefore, composed of the same elementary structure, the former being 
especially intended to afford resistance to violence with as little weight as 
is consistent with its office, for which reason it is hollowed into a tube ; 
while the latter is enlarged as much as possible without unnecessarily 
adding to its weight, the problem being solved by its development in a 
cellular form. 
The Periosteum is a dense fibrous membrane which covers every part of 
the surface of the bones, excepting their extremities when they enter 
into the composition of a joint, its place being then occupied by cartilage. 
(See Joints.) When this membrane covers the bones of the skull it is 
called pericranium, and when it invests the cartilages of the ribs it 
receives the name perichondrium. It is full of blood-vessels, especially in 
the young, and they freely communicate with those of the surrounding 
soft parts. Hence it is extremely liable to inflammation, either caused 
by injury to itself or to the parts which cover it. 
The marrow, or medullary substance, is contained in the cavities formed 
within the bones, being of a yellow colour and oily nature in the shafts 
of the long bones ; and more or less red, from the admixture with blood, 
in the flat and irregular bones, and in the heads of the long bones. It is 
contained within the areolar meshes of a membrane, which lines these 
cavities, answering to the periosteum, which has been already described. 
This medullary membrane is of excessive tenuity, and is composed of blood- 
vessels ramifying in fine cellular tissues. The use of marrow in the animal 
economy is not very clearly demonstrated. 
In the embryo, all the bones originally exist in the state of cartilage, 
being soft and flexible. By degrees vascular canals are developed within 
its substance, by the union of its cells in rows. These concentrate 
towards some one or more points, which in a long bone are one in the 
centre of the shaft and one at each extremity. Starting from this point 
(punctum ossificationis), fibres run out, embracing clusters of cells, and 
sending branches between the individuals composing each group. In 
this manner the network, characteristic of bone, is formed, the cells 
uniting to form the permanent areole and Haversian canals. At first the 
contents of the cells are transparent, then granular, and finally opaque, 
from the pressure of amorphous mineral matter. The several ossified 
portions are quite distinct for a long time in the young animal, and may 
readily be separated by boiling or maceration. 
OF THE SKELETON IN GENERAL. 
THE NAME SKELETON has been given from the Greek word oxéAdw (to 
dry), it being the only part of the body which will bear dessication without 
change of form. In the vertebrata it is an internal bony framework, but 
in the crustacea it invests the soft parts, and forms an insensible covering 
to them, while at the same time it serves the purpose of locomotion. In 
both these divisions of the animal kingdom the skeleton forms a series of 
arches or rings, capable of moving on each other, but so firmly attached 
as to secure protection to the important organs contained within them. 
Tn the horse, as in all the higher mammalia, these rings or arches are 
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