SKELETON OF THE HORSE. 17 



circumference by deposits of new hone on its external surface, 

 derived not from eartilage, but from membrane — the. inner layer 

 of the periosteum, which has been termed the osteogenic mem- 

 Irane. .In the periosteum there are two layers or strata — an 

 outer, strong and fibrous ; an inner, soft, and containing osteo- 

 blastic cells, which, after undergoing certain changes, produce 

 layers of new bone; the process is termed intramembranous, 

 as opposed to intracartilaginous ossification. This goes on 

 till the bone has reached its full development, when Haversian 

 systems cease to be formed, and then, as a climax, the peri- 

 pheral lamellae are formed, and the medullary canal enlarges 

 by internal absorption. 



In flat bones, ossification usually radiates from a centre, and 

 is directed by the membrane investing each surface of the bone; 

 some of them possess numerous apophyses. Ossification is 

 completed in some bones much earlier than in others, and at 

 birth those which are required for support and progression are 

 farthest advanced. 



The bones of the cranial vault are developed, not from carti- 

 lage, but from membrane. In early foetal life the brain is 

 covered by two membranes, closely united— viz., the pericranium 

 and dura mater ; between these, bone becomes developed from 

 radiating ossific centres. 



The Skeleton. 



The internal framework on which the animal body is built is 

 termed the Endoskeleton, while the outer casing which deter- 

 mines the shape and appearance of the animal is the Exoskele- 

 TON ; in some of the lower animals the latter assumes the form of 

 a calcareous case, in others it is scaly, in others again, bony. 

 The exoskeleton of the higher animals consists of the skin and its 

 appended modifications, which will be described in a future sec- 

 tion of the work. By the term skeleton is generally understood 

 the endoskeleton, or bones of an animal, held in their relative 

 positions, or articulated by wires or screws, in which case all the 

 soft tissues are first removed, and the preparation is termed an 

 artificial skeleton : but if those soft structures, whicli, in the 

 living animal, join the bones together, are allowed to remain, 

 dried, and preserved, then the preparation is a natural skeleton. 



As the skeleton has two sides or halves, the majority of the 

 bones exist in pairs. There is, however, a central chain of single 

 bones, formed by the back bones or vertebrae and the median 



C 



