GENERA L PRINCIPLES A PPLICABLE TO THE STUD Y OF ALL TEE BONES. 23 



If the phenomena of growth were not counterbalanced by those of absorption, 

 long bones would acquire an enormous thickness and weight. But in the first 

 period of life, in proportion as new layers are applied to the surface of bones, the 

 deeper parts — those occupying the axis of the diaphysis — disappear by absorption. 

 In this way the medullary canal is formed, and a just proportion established 

 between the volume and weight of the skeleton. 



When the bones are completely formed, during the entire adult period the 

 process of destruction is equal to that of growth, so that their weight and com- 

 position does not vary. At a later period, absorption is greater than the forma- 

 tive force, which in old age is altogether in abeyance ; so that the organic 

 matter of the bones is rarefied, and these lose their elasticity and gain in 

 fragility. 



It has been remarked that the development of the bones is subordinate, in 

 a certain measure, to the activity and resistance of the organs in their vicinity. 

 Lesshoft and Popoff beheve that the development of a bone is in proportion to 

 the activity of the neighbouring muscles, that the pressure of external organs- 

 such as an aponeurosis — may produce torsion in a bone and retard its growth, 

 so that its greatest thickness will correspond to the point where the surrounding 

 Ijf^ resistances are at a minimum. Oilier, however, has noted that the long bones 

 i^*^"^ become more elongated when they do not sustain pressure on their extremities. 



2. Flat Bones. — These bones have sometimes only one primary nucleus of 

 ossification, placed in the centre. They grow by the increase of this nucleus, 

 which gradually invades the mass of the bone in radiating from the centre towards 

 the periphery. When they have several nuclei, some of these are at the margin, 

 and in such a case the bones increase by ossification of the marginal efpiphyses 

 and growth of the central nucleus. 



Flat bones increase in thickness by the formation of sub-periosteal layers, 

 and by the development of the spongy tissue between the two compact plates. 



3. Short Bones. — These grow in thickness by the progressive ossification of 

 the osteogenic layer of the periosteum ; and in length by ossification of the 



• epiphysary cartilages, when they possess complementary nuclei. 



Nutrition of Bones. 



The experiments which consisted in feeding young animals with madder, and 

 afterwards examining their osseous system, have for a long period demonstrated 

 the nutrition of bones. When bones cease to grow, nutrition becomes less active ; 

 but it is evident that it does go on, in order to maintain the organic matter of 

 the osseous tissue in a proper condition. 



The abundance of vessels carrying blood to all parts of the bone tissue, alone 

 suffices to prove the existence of a nutritive movement in these apparently stony 

 organs. 



