250 THE HORSE. 



Structure, the fornaer being especially intended to afford resistanea 

 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. 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 color and oily nature 

 in the shafts of the long bones ; and more or less red, from the ad- 

 mixture 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 perios- 

 teum, which has been already described. This medullary mem- 

 brane is of excessive tenuity, and is composed of blood-vessels rami- 

 fying 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 car- 

 tilage, 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 ex- 

 tremity. 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 net- 

 work, characteristic of bone, is formed, the cells uniting to form 

 the permanent areolae 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 GENEEAL. 



The name skeleton has been given from the Greek word 

 ffxiUaj (to dry), it being the only part of the body which will bear 

 desiccation without change of form. In the vertehrata it is an in- 

 ternal bony framework, but in the Crustacea it invests the soft 

 parts, and f Drms an insensible covering to them, while at the same 



