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CHAPTER IV. 

 THE SKELETON OF THE FROG. 



The skeleton, which forms the hard internal parts of the frog, 

 is composed partly of cartilage and partly of bone. It forms a 

 framework giving definite shape to the body, and precision to 

 the movements; and serves also to protect from injury some of 

 the more important and delicate organs, notably the central 

 nervous system, the sense organs and the heart. In the early 

 stages of its development the skeleton consists entirely of car- 

 tilage : in the adult this primary cartilaginous skeleton is replaced 

 to a greater or less extent by bone. Bone may also be developed 

 in places where there was no pre-existing cartilage, and is then 

 called 7nemhrane-hone, in contradistinction to the former kind, 

 or cartilage-bone, which replaces cartilage. Membrane bones 

 arise in the first instance as ossifications in the dermis or deeper 

 layer of the skin : in many fish they retain this primitive 

 position, but in the frog and most higher vertebrates they sink 

 below the skin and graft themselves on to the more deeply placed 

 cartilaginous skeleton. Cartilage may also become calcified, 

 i.e., have calcareous salts deposited in its matrix, without in any 

 way taking on the character of true bone. 



The skeleton may conveniently be divided into (1) the axial 

 portion, including the skull and the vertebral column : and 

 (2) the appendicular portion, including the limbs, and the 

 limb-girdles which attach them to the body. 



Examine the ^jrej^areci skeletons, and make careful drauings to 

 scale of the several parts. In your drawings colour the cartilage 

 blue, the cartilage hones yellow, and the membrane hones white or 

 red. Prepare skeletons for yourself by soaking the 2^<^^'ts in hot 

 water, and carefully brushing away the soft tissues until the 

 skeleton is clean. 



