SKELETON OF THE HORSE. 17 



voung animals ; they disappear at variable periods, the portions 

 of bone hitherto imperfectly united becoming consolidated into 

 one firm mass. The bone increases in length by growth of the 

 unossified ring, uniting the shaft and epiphysis, until the ring fills 

 up, when growth is completed. Should an epiphysis unite with 

 the diaphysis prematurely, by acceleration of the process of ossifi- 

 cation through disease, growth being thus arrested, the bone will be 

 shorter than its fellow. The shaft of a long bone increases in 

 circumference by deposits of new bone on its external surface, 

 derived not from cartilage, but from membrane the inner layer 

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

 brane. 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 lamellas 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 



c 



