CERTAIN TISSUES. 609 



bone. The main artery, whence the others are derived, for the 

 shaft of the hone, enters by the nutrient foramen on the sur- 

 face, and toward the center. 



The bone-corpuscles (Fig. 446), answering to the connective- 

 tissue cells, are nutritive and formative after a considerable 

 portion of the tissue has become the seat of the deposit of lime- 

 salts. Bone is a living tissue, though in a less degree than 

 most others ; but it is only by bearing these relations in mind 

 that its function in the support of the soft parts of an animal, 

 and especially as constituting the essential levers of its locomo- 

 tive mechanism, can be understood. 



Cartilage. — In the earliest stages of an animal's existence 

 the bones are represented by cartilage, and at all periods of its 

 existence this structure forms those elastic pads that cover its 

 articular surfaces, and shield the bones and the entire animal 

 from undue concussion. The kind of cartUage that covers the 

 extremities of the long bones, known as articular, is character- 

 ized by abundance of cells lying in a homogeneous bed or mar 

 trix (Fig. 447). 



Fibro-cartilage (Fig. 448) abounds in fibrous tissue, some 

 elastic fibers, characteristic cells, etc., and is found between the 

 bodies of the vertebrae and in similar situations, as well as in the 

 epiglottis, the ear, etc. 

 89 



