THE RABBIT 



53 



Around the margin of any mass of cartilage is a layer of 

 connective-tissue called perichondrium. This contains many 

 white fibres. Cartilage is a comparatively unimportant 

 tissue in the full-grown rabbit. To see it in its greatest 

 development we must go to such a type as the dogfish. 

 Here it forms the whole skeleton, affording the compara- 

 tively rigid basis for the attachment of muscles, without 

 which rapidity and accuracy of motion would not be possible. 

 In the rabbit, cartilage forms the main skeleton at an early 

 stage of its development, but it is afterwards replaced by 

 the more efficient tissue called bone. 



9. Bone differs from cartilage primarily in the com- 

 position of the matrix it contains not chondrin but gelatin 

 and this is im- 

 pregnated with 

 various inorganic 

 salts, of which 

 calcium phos- 

 phate is the 

 chief ; calcium 

 carbonate and 

 fluoride, and 

 magnesium phos- 

 phate, being also 

 present. This 

 matrix is not 

 readily per- 

 meable to the 

 blood-plasma, 

 and hence we 

 find a complex arrangement for bringing the blood to all 

 parts of the bone. If we examine such a bone as the 

 femur (thigh-bone), we find it to be, roughly, a hollow 

 cylinder the hollowness diminishing the weight without 

 lessening the strength. Running, for the most part longi- 

 tudinally, through the bone there are a great number of 

 canals known as Haversian canals, each of which contains 

 during life an artery and a vein. These are branches 

 of a main artery and vein that pierce the shaft at one 



Fig. 16. BONE. 

 Transverse section. 



