THE STUDY OF THE BONES. 13 



(c) A shott bone offers for description a variable number of faces, and 

 plane or salient angles, which are often neglected because of their triiiing 

 importance. 



Internal Conformation of Bones. 



Sections made in various directions through the substance of bones show 

 that their internal conformation varies, acoording as they belong to the 

 category of long, flat, or short bones. The diaphysis of long bones is 

 hollowed out into a large fusiform cavity ; this is the medullary canal. 

 This canal is absent in the flat and short bones. Its walls are formed by a 

 very dense bony tissue, whose pores are scarcely visible to the naked eye, 

 and which is called the compact substance. The extremities of long bones 

 are surrounded by a thin layer of compact substance, while the remainder 

 of their mass is constituted by the spongy substance — bony tissue channeled 

 into cells, or very large areolte, which freely communicate with each other. 

 (Beticulated bony tissue is but another form of spongy substance, the only 

 difference between the two consisting in the cells or meshes of the first 

 being formed of intercrossed osseous fibres, while those of the second are 

 formed of lamellcB.) The medullary canal, and areolas of the spongy tissue, 

 are filled by a cellulo-fatty substance, the marrow (or medulla). The flat 

 bones are constituted by a layer of spongy tissue placed between two laminsB 

 of compact substance. In the flat bones of the cranium, the two layers of 

 compact tissue are termed the vitreous tables, while the cells of the spongy 

 tissue are designated diploe. In certain points of their extent, the spong3'- 

 substance disappears, and then the bone is found to be composed of a single 

 lamina of compact tissue. The short bones have a nucleus of spongy 

 substance, enveloped in a layer, more or less thick, of compact tissue. 



The compact substance of the bones being very resisting, is found in all 

 those situations which have to sustain violent efforts. The spongy substance 

 is very light and bulky, and is met with in the widened portions of the 

 bones, to which it affords increased size without adding sensibly to their 

 weight. 



Structure of Bones. 



Bones are formed of a proper tissue, covered externally by a particular 

 membrane, the periosteum, and occupied internally 

 by the medulla, vessels, and nerves. 



Proper tissue. — The texture of the proper tissue 

 of bones varies slightly in the compact and spongy 

 substance. 



The compact tissue is composed of a funda- 

 mental substance, which is amorphous, or slightly 

 granular, white, and more or less opaque, accord- 

 ing to the thickness it offers. 



This fundamental substance is penetrated by 



infinite number of vascular canaliculi, known 



the Haversian canals. These canals, which 

 from l-2500th to l-200th of an inch in 

 diameter, are parallel to each other and to the 

 larger axis of the bone; they frequently com- 

 municate by' transverse branches. The most 

 superficial open on the surface of the bone, be- 

 neath the periosteum, and the deepest into the 



an 

 as 

 measure 



VERTICAL SECTION OF BONE, 

 showing the network of 

 Hayersian canals. 



