414 CHORD AT A. 



say in the mechanical centre, and thence to form bone to 

 either end. But bone is a harder and more resistant substance 

 than cartilage, and it is often more to the advantage of the 

 organism that the parts which are subjected to special strain 

 should first be ossified. Hence the ends of the long bones, 

 which form the joints, and very often other parts, such as 

 trochanters and tuberosities acting as points of attachment 

 for muscles or tendons, have separate centres of ossification. 

 When the cartilage ceases to grow, then the ossification 

 proceeding from each centre, the bony elements meet and a 

 single complete bone results. If, as in mammals, the bony 

 elements are separated for a long time by a thin layer of 

 growing cartilage, then the elements are separated in the 

 dried skeleton by "sutures" and may fall apart. Hence 

 the caps or epiphyses already referred to. But the final 

 result is a single bone of the same size and shape as the 

 cartilage. 



In many cases the single piece of cartilage may be re- 

 placed permanently by two or more bones with a joint 

 between them. Cartilage is elastic, and a piece of cartilage 

 may therefore " give " to certain strains, by virtue of its elas- 

 ticity, sufficiently to dispense with the necessity for a joint. 

 Bone, however, is far more rigid, and hence a single elastic 

 cartilage, such as the palatoquadrate bar, is replaced by at 

 least three bones — the palatine, pterygoid and quadrate — 

 which are more or less movable on each other. The replace- 

 ment of the hyomandibular cartilage of lower types by three 

 (or four) ossicles of the ear is probably another instance. 



The skeleton can be conveniently considered under two 

 heads : — i. The axial skeleton, skull and vertebrae. 2. The 

 appendicular skeleton, limbs and limb-girdles. 



Axial. — The skull has a double origin, being really 

 formed of two parts which are almost entirely distinct in 

 the fishes. These are (i) cranium; (2) the visceral arches. 



The cranium arises essentially as a protecting mass to 

 the underlying brain, and the visceral arches arise primarily 

 as strengthening bars between the branchial clefts. The 

 first two of these arches alone take any part in the formation 

 of the skull. 



(i) The Cranium. — In the earliest stages the brain is 

 enclosed on all sides by a membranous sheath which also 



