THE CONSTRUCTION OF THE HORSE 131 



down in the form of cartilage, in which condition they 

 appear prior to the birth of the foal. Their transforma- 

 tion into bone is spoken of as ossification, a process which 

 commences at certain centres in each bone and gradually 

 extends until the whole structure becomes converted 

 into bone. The bones of the limbs differ from those in 

 the rest of the framework ; most of the former have a 

 medullary canal filled with red marrow, whilst the ends 

 of the bone have in their interior cancellated bony tissue 

 filled with yellow marrow. These are spoken of as long 

 bones in contradistinction to short, flat, and irregular 

 bones participating in the construction of the rest of 

 the skeleton. 



The head is composed of a number of bones which, soon 

 after birth, become more or less ossified, and the only 

 movement, or rather, we ought to say, freedom of move- 

 ment, is that executed between the upper and lower jaw. 



The neck is composed of seven segments. These are 

 the cervicle vertebrae, and the bone next to the head is 

 called the atlas, and the one following it the axis. The 

 articulation between the atlas and the head gives the 

 horse its nodding movement, whereas that between the 

 atlas and the axis confers a rotatorial one. The rest of 

 the vertebrae articulate amongst themselves. Each 

 vertebrae is hollow, forming a segment of a canal, so that 

 when all the vertebrae, extending from the atlas to the 

 bones of the tail, are united together, they form a canal 

 — the nural canal — which lodges the spinal cord, and from 

 which the spinal nerves originate, to be subsequently 

 distributed to various organs and tissues throughout the 

 body. 



The spinal cord commences at the base of the brain in 

 a structure known as the medulla oblongata, and from 

 the latter numerous nerves arise. The spinal cord is an 

 extremely delicate structure composed of white and grey 

 matter, and it is invested by three membranes correspond- 

 ing to those covering the brain. The nural canal, in which 



