46 ANIMAL PHYSIOLOGY. 



the body of tlie lower jaw elongates by that means sufficiently 

 to make room within it for the permanent 

 teeth, and an expansion of the same sort 

 takes place in the frontal bone between the 

 frontal eminences and the margins of the 

 orbits. But the principal elongation of the 

 bones of the limbs is provided for by the 

 extremities of the bones being furnished 

 with separate epiphyses or supplementary 

 centres of ossification, between which and 

 the shaft there remains, as long as the growth 

 of the bone continues, a thin plate of carti- 

 lage, which is as rapidly growing intersti- 

 tially as it is converted into bone at its 

 surface. So, also, the bones of the skull 

 expand principally by additions at their 

 edges, and when premature obliteration 

 (synostosis) of any suture occurs, it causes 



p- 25 TIBIA at arres ^ ^ growth at right angles to the 



18 years of age, suture, compensated for by additional growth 

 showing the su- in other directions, producing a variety of 

 perior and in- anomalous forms of skull. 



fenorepiphyses. 25. The Joints or Articulations, by 

 which the different bones are joined together, may be 

 divided primarily into movable and immovable. 



All the bones of the skull, with the exception of the lower 

 jaw, are united together by immovable articulations or sutures, 

 many of them rendered firmer by the doyetailing of compli- 

 cated serrations of the articulating edges. Such articulations 

 are serviceable for purposes of growth, as has been already 

 explained. 



Movable articulations are divisible into complete or per- 

 fect, and incomplete or imperfect. 



Incomplete joints are those in which the opposed sur- 

 faces of bone are united by intervening substance of a 

 yielding description ; and the most notable example of this 

 mode of union is found in the vertebral column. The arches 

 of the vertebrae are united by pairs of complete joints, but 

 their bodies, the parts through which the weight of the trunk 

 is principally conducted, are joined by intervertebral discs, 



