CHARACTERISTICS OF THE ANIMAL BODY 29 



trachea, the epiglottis, and the covering at the ends of long bones 

 are examples of cartilaginous tissue which persists in the mature 

 animal. 



3. Bone tissue — the skeleton or great supporting framework of 

 the mature animal — is formed by a deposit of earthy salts. Bones 

 are composed of organic and inorganic matter. The former consti- 

 tutes about one-third the total amount, and when freed from the 

 inorganic substances is known as ossein, a form of gelatin. This, 

 may be demonstrated by soaking a fresh bone in weak hydrochloric 

 acid to dissolve out the mineral portions (decalcification). The 

 organic matter gives bone its elasticity and tenacity. The inor- 

 ganic or mineral matter constitutes the other two-thirds and is in 

 the form of — 



Calcium phosphate 57.35 per cent. 



Calcium carbonate 3.85 per cent. 



Magnesium phosphate 2.05 per cent. 



Sodium salts 3.45 per cent. 



66.70 per cent. 



The inorganic matter renders bone hard and dense. This combina- 

 tion of animal and mineral substances gives solidity to the bones, 

 yet elasticity enough to prevent fracture under ordinary circum- 

 stances. It is very difficult to break a green bone on this account. 



Histologically, bone is found to be composed of a series of bars 

 and plates arranged so that blood-vessels and nerves may freely 

 pass between them. This arrangement is so uniform that it has 

 been given the name Haversian system. Each bone represents 

 a great number of such systems. 



On sectioning a bone there are revealed two varieties of bone 

 texture: the outer or compact tissue, and the inner, spongy or 

 cancellated tissue (Fig. 2). 



Corn-pact hone tissue is dense, close grained, and varies in thick- 

 ness. In long bones it occurs in greatest amount in the shaft, the 

 center of which is almost exclusively composed of it. Toward 

 the ends of long bones it gradually diminishes in thickness until at 

 the extremities it forms only a shell-like covering. On account of 

 its resistance it is found in abundance wherever great strain is 

 placed on a bone. In the cannon bone it is thick in the center, but 

 thicker in front than behind, and on the inside than outside, in 

 correspondence with the Unes along which the greatest weight of 

 the body falls. 



