THE ANATOMY OF THE HORSE 345 



minute cells (corpuscles of Purkinjo). TIk; l);isi,s subsiaiicc^ is |).arti;Llly 

 fibrous and slightly liimollatcd, the layers being concentric in l(jng bones and 

 parallel in flat ; it is traversed in all directions (more especially in the long 

 axis, where there is one) by canals (Haversian canals), which frequently 

 branch and inosculate, giving passage to vessels and nerves. In certain 

 situations the lamelhu separate, and leave between them spaces of various 

 sizes, called cancelli. Besides entering into the composition of the basis 

 substance, the lamelhc are collected concentrically round the Haversian 

 canals, the boundaries of which they form, generally to the extent of ten to 

 fifteen layers. l]oth the compact and spongy tissues are, therefore, com- 

 posed of the same elementary structure, the former being especially intended 

 to alTord resistance to violence with as little weight as is consistent with its 

 oflice, 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 fd)rous 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 cartilag(!S 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 containc^d 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 tlu; 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 ossiji- 

 cationif^), 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 areolic 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 ((uite distinct 

 for a long time in the young animal, and may readily be separated by boiling 

 or maceration. 



