STRUCTURE OF BONE. 249 



fatty nature, the marrow^ and are pierced in various directions by 

 blood-vessels and nerves. 



THE PROPER TISSUE of the bones is made up of two distinct 

 substances, either of which may be removed by artificial means, 

 leaving the other entire. If, for instance, a bone is submitted to 

 the heat of a furnace, it retains its shape and rigidity, but becomes 

 much whiter in color, and is rendered extremely brittle. In fact, 

 the mineral salts entering into its composition are left, but the ani- 

 mal matter binding them together is completely decomposed and 

 carried off in a gaseous form. On the other hand, by immersing 

 a bone for two or three weeks in diluted hydrochloric acid, the 

 earthy salts are dissolved, while the animal matter is untouched. 

 Here the bone retains its original shape, but it is soft and flexible; 

 arid instead of presenting its usual opaque yellowish-white color, 

 it is semi-transparent, and resembles the ordinary gelatine of the 

 shops. According to Berzilius, bone is chemically composed of 

 the following constituents namely, cartilage, reducible to gelatine 

 by boiling ; blood-vessels ; phosphate of lime ; carbonate of lime ; 

 fluate of lime ; phosphate of magnesia ; soda and chloride of 

 sodium. 



Considered mechanically, the bones form the framework of the 

 animal machine. In the limbs they are hollow cylinders, admirably 

 fitted by their shape and texture to resist violence and support 

 weight. In the trunk and head they are flattened and arched, to 

 protect the contents of the cavities they form, and to provide an 

 extensive surface for the attachment of muscles. In certain situa- 

 tions their exterior is raised into projections called processes, which 

 serve as levers for the muscles to act upon ; in others they are 

 grooved into smooth surfaces for the easy gliding of tendons, when 

 these are stretched between the fleshy part of a muscle and one of 

 its attachments. Lastly, they sometimes present a large hollow for 

 the lodgment of the belly of a muscle, as in the case of the scapula. 



When microscopically examined, bone is seen to be made up of 

 a dense and homogeneous substance (basis substance), in which 

 are numberless minute cells (corpuscles of Purkinje). The basis 

 substance is partially fibrous and slightly lamellated, the layers 

 being concentric in long bones and parallel in flat; it is traversed 

 in all directions (more especially in the long axis, where there is 

 one) by canals (Haversian canals), which frequently branch and 

 inosculate, giving passage to vessels and nerves. In certain situa- 

 tions the lamellae separate, and leave between them spaces of va- 

 rious sizes, called cancelli. Besides entering into the composition 

 of the basis substance, the lamellae are collected concentrically 

 round the Haversian canals, the boundaries of which they form, 

 generally to the extent of ten to fifteen layers. Both the compact 

 and spongy tissues are, therefore, composed of the same elementary 



