344 THE STRUCTURE OF BONE 



The proper tissue of the bones is made up of two distinct substances, 

 cither 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 rigidit}^ but becomes much whiter in colour, and is rendered 

 extremely brittle. In fact, the mineral salts entering into its composition 

 are left, but the animal matter binding them together is completely decom- 

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

 a Ijone for two or three weeks in diluted hydrochloric acid, the earthy salts 

 are dissolved, while the animal matter is untouched. ITere the bone retains 

 its original shape, but it is soft and flexible ; and instead of presenting its 

 usual opaque yellowish-white colour, it is semi-transparent, and resembles the 

 ordinary gelatine of the shops. According to Berzillius, 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 situations 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. These differently shaped bones may, 

 therefore, be classed under the following three heads : — 



1st. The long hones consist of the humerus, radius, ulna, femur, tibia, and 

 fibula ; the metacarpal and metatarsal bones (called, in horsemen's language, 

 the cannon bones), the phalanges (pastern bones), and the ribs. These bones 

 ax'e all divisible into a central cylindrical shaft, and two heads or extremi- 

 ties. The shaft is usually of a prismoid form, dense in texture, and 

 presenting a longitudinal tube in the interior, called the medullary canal, 

 which contains marrow. The heads are broad, to articulate with the next 

 adjoining bones, and are covered with a thin layer of cartilage, which will 

 be described in the chapter treating of the joints. Their outer surface is a 

 liard osseous layer, within which is a mass of cells containing red medullary 

 matter, to be presently described. 



2nd. The Jlat hones are composed of two layers of dense tissue, one on 

 each surface, having between them another of a cellular nature, called the 

 diploe. As a matter of course, from their shape, they have surfaces, borders, 

 and angles ; in addition to which they have projections, called processes, of 

 various shapes. They consist of the chief bones of the head, the scapula, and 

 pelvis. 



3rd. The irregxdar hones comprise the lesser bones of the head and face, the 

 vertebrae, sacrum, sternum, carpal and tarsal bones, the sesamoid bones, the 

 bones of the foot, and the patella?. They resemble the flat bones in their 

 structure. 



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

 and homogeneous substance (basis substance), in which are numberless 



