OSTEOLOGY 5 



the slightest degree. The ailimal and earthy matter, as they 

 form bone, are intimately combined. 



The proportion of about one-third of animal matter to about 

 two-thirds of earthy matter apphes to the healthy adult. In 

 young children the relative amount of animal matter is much 

 greater, so that in them the bones are ver\' tough and elastic, 

 but not very hard, and therefore not capable of bearing much 

 weight. In old age there is a relatively large amount of earthy 

 matter present. The bones of old persons, therefore, are very 

 hard and brittle, but not ver^ tough and elastic ; hence the f requeue}^ 

 of fractures in old persons from slight causes. 



Structure of Bone.— There are two varieties of osseous tissue, 

 namely, compact, and spongy or cancellated. Compact osseous 

 tissue is so named because its constituents are so closely packed 

 together that the bone appears to the naked eye {macroscopic) 



r:G. 



-Structure of Compact B. 



A, Longitudinal section, sho\v-ing Haversian canals ; B, Transverse 

 section, showng Haversian systems. 



to be dense and close like ivory. Spongy or cancellated osseous 

 tissue, on the other hand, presents an open porous appearance like 

 a sponge. It is called cancellated because it resembles lattice- 

 work. These two varieties of osseous tissue merge very gradually 

 into one another. 



When compact bone is viewed under the microscope in thin 

 transverse section it presents a number of small round or oval 

 openings, and when viewed in thin vertical section it presents 

 short longitudinal tuhes, called Haversian canals. These pervade 

 every part of compact bone, and the innermost open into the 

 marrow canal of long bones, whilst the outermost open by minute 

 orifices on the external surface. They range in diameter irom ywoxj 

 to 2^ inch, the average being 5^ inch. The smallest lie nearest 

 the external surface, and the largest are nearest the marrow canal. 

 They are very short and longitudinal in direction, and they com- 



