315 



CHAPTER XII. 



THE STEUCTUEE OF THE HOESB, 



In the structure of a craniote animal there are two main 

 divisions to be studied — namely, the skeletal structure and the 

 general internal organisation. Every craniote animal has an 

 internal skeleton. In the lowest forms, the fish, this skeleton 

 may be soft and composed only of gristle-like cartilage; or it 

 may be completely ossified or bony, as in the horse. Bony or 

 osseous tissue is partly preceded by cartilaginous tissue : the 

 bones of young animals are quite soft as compared with the 

 bones of the adults, because the calcareous salts which form the 

 bone are not yet fully deposited. Bones are also formed by 

 calcareous deposits in membranes. 



Skeleton of the Hoksb. 



The skeleton of the horse (fig. 181) will be seen, in common 

 with that of all the Craniota, to be easily divisible into two 

 parts — the axial part or spinal column and the cranium, and 

 the so-called appendicular portion, composed of the limbs and 

 their arches. The limb-arches unite the limbs to the central or 

 axial skeleton, and with these arches the limbs articulate. 



The trunk or vertebral column is composed of a num- 

 ber of bony rings — the vertebrae. These bony vertebrae are 

 placed in a longitudinal direction. There are five divisions 

 of these bones, which are known as cervical, dorsal, lum- 

 bar, sacral, and coccygeal vertebrae. The cervical are found 



