284 



BIOLOGY AND HUMAN LIFE 



cartilage cells at various points. Some of these cartilage cen- 

 ters in time become bones ; the cartilage at the ends of certain 

 bones does not become completely ossified (that is, changed into 

 bone) until maturity is reached (see Fig. 94). Some cartilage 

 structures never become bony. In some of the lowest fishes 

 the entire skeleton remains permanently cartilage. 



The long bones have hollow shafts containing marrow. Where 

 two bones move against each other the enlarged ends furnish in- 

 creased surfaces. Here the bones are spongy on the inside. 



Thus the skeleton combines me- 

 chanical strength with relatively 

 light weight. Among birds the 

 bones are rather more compact 

 but contain larger holl.ows and 

 are relatively lighter. 



Through cell division in sur- 

 face layers, which make up the 

 periosteum, the bone grows in 

 thickness ; and it is also from 

 these tissues that injuries are re- 

 paired by the formation of new 

 bone. Bones increase in length 

 through cell division in special 

 sections between the shaft and 

 the head at each end. As age advances, these grow^ing areas 

 become hardened and further growth is impossible. 



The large proportion of cartilage in the skeleton of an infant 

 accounts for the softness and flexibility of the young organism 

 as compared with the adult or aged. With increasing propor- 

 tions of lime the bones become more brittle. It is accordingly 

 important to prevent pressures and postures in childhood that 

 may lead to distorted or misshapen organs. Primitive peoples 

 used to press the heads of babies out of shape, and some used to 

 squeeze the feet out of shape. Even in comparatively modern 

 times, in Europe, children have been crippled or twisted in 

 order to serve as bait for charity in the business of begging. 



Fig. 137. Growth of bone 



Living cells, a, deposit lime around 



themselves but remain connected by 



tiny canals with one another and with 



the circulation of lymph, b 



