CONNECTIVE TISSUES 49 



lamella?, arranged as Haversian, interstitial, peripheral, and 

 medullary lamellae (fig. 15). 



One important function performed by the cartilage is in 

 bringing about the increase in length of the bones. As 

 growth progresses, the cartilage grows in length, and the 

 formation of bone outside the cartilage spreads to each end, 

 and thus the shaft of the bone is formed. But, in addition 

 to the centre of ossification in the shaft — the dicqjhysis, one 

 or more similar centres of ossification form at each end 

 of the bone. These are the epiphyses. Between these and 

 the diaphysis a zone of actively growing cartilage exists until 

 adult life, when the bones stop lengthening. In this zone, 

 the cells arrange themselves in vertical rows, divide at right 

 angles to the long axis of the bone and form cartilage. 

 This cartilage, as it is formed, is attacked by the bone-form- 

 ing changes at the diaphysis and epiphyses. But the 

 amount of new cartilage formed is at first proportionate to 

 this, and thus a zone of growing cartilage continues to exist 

 until early adult life, when epiphyses and diaphysis join and 

 growth in length is stopped. The rate and extent of this 

 growth of the cartilage determines the length of the limbs. 

 It is influenced by many factors, such as the general nutrition 

 of the animal, and also by the influence of the internal 

 secretions of various structures, such as the thyreoid and 

 pituitary body (see pp. 595, 599). 



(2) Chemistry. — The composition of adult bone is 

 roughly as follows : — 



Water, 10 per cent. 

 Solids, 90 per cent. 



Organic, 35 per cent. — chiefly collagen. 

 Inorganic, Q>b per cent. 

 Calcium phosphate, 51. 

 „ carbonate, 11. 

 fluoride, 0-2. 

 Magnesium phosphate, 1. 

 Sodium salts, 1. 



The most important points are the small amount of water, 

 4 



