22 ANATOMY FOK NUKSES. [CHAP. Ill 



follows that the innermost ones must run into the Haversian 

 canals, so that there is a direct communication between the 

 blood in these canals and the cells in the lacunae. In this way 

 the whole substance of the bone is penetrated by intercom- 

 municating channels, and nutrient matters and mineral salts 

 from the blood in the Haversian canals can find their way to 

 every part. 



The mineral or earthy substance which is deposited in bone, 

 and which makes it hard, amounts to about two-thirds of the 

 weight of the bone. It consists chiefly of phosphate of lime, 

 with about a fifth part of carbonate of lime, and a small portion 

 of other salts. The soft or animal matter consists chiefly of 

 blood-vessels and connective tissue, and may be resolved by boil- 

 ing almost entirely into gelatine : it constitutes about one-third 

 of the weight of the bone. This proportion of animal and earthy 

 constituents is, however, subject to variations. In children the 

 animal matter is apt to predominate, and we find that their 

 bones are more liable to be bent when injured, or only partially 

 broken, as in the typical "green-stick" fracture. Again, in aged 

 people the earthy constituents increase, the animal matter be- 

 comes deficient in quality and quantity, and consequently their 

 bones are brittle, and more liable to fracture. 



In the re-union of fractured bones new bony tissue is formed 

 between and around the broken ends, connecting them firmly 

 together; and when a portion of bone dies, a growth of new 

 bone very generally takes place to a greater or less extent, and 

 the dead part is thrown off. The periosteum is largely con- 

 cerned in this process of repair ; for if a portion of the perios- 

 teum be stripped off, the subjacent bone will be liable to die, 

 while if a large part or the whole of a bone be removed, and 

 the periosteum at the same time left intact, the bone will 

 wholly, or in a great measure, be regenerated. 



In the embryo the foundation of the skeleton is laid in cartilage, or in primi- 

 tive membranous connective tissue, ossification of the bones occurring later. 

 The hardening or ossification of the bones is accomplished by the penetration of 

 blood-vessels and bone-cells, called osteo-blasts, rfrom the periosteum. As 

 they penetrate into the cartilaginous or membranous models, they absorb the 

 cartilage and connective tissue and deposit the true bone tissue at various 

 points until they form the particular bony structure with which we are familiar. 



