16 



TEE BONES. 



the pores of which 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 medullary spaces. { Reticulated 

 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 lameUce.) The medullary 

 canal, and areolae of the spongy tissue, are filled by the marrow (or medulla). 



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

 plates 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 spongy 

 substance disappears, and then the bone is 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 hght when compared with its volume, and is met with in the wider 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. 



A. Proper tissue. — The elements of the proper tissue of bone are always and 

 everywhere the same ; the texture alone is modified in 

 the compact and spongy substance. 



Everywhere the bone tissue is composed of a funda- 

 mental substance, which is amorphous or slightlj 

 granular, white, and more or less opaque, according to 

 the thickness it offers. This fundamental substance is 

 penetrated by an infinite number of vascular canaliculi 

 [bone cavities), with prolongations {hone canaliculi), 

 which contain cells {bone cells). The cavities and their 

 contents are named osseous corpuscles or osteoplasts. In 

 a dried plate of bone, the corpuscles appear dark when 

 viewed by reflected light, white and shining by direct 

 light. 



In the spongy tissue, the bone corpuscles, anasto- 

 mosing by the ends of their canahcuh, are distributed 

 throughout the lamellte of fundamental substance, which 

 is intercrossed in such a way as to circumscribe the 

 numerous medullary spaces. 



In the compact tissue, the corpuscles are regularly distributed in the 

 substance of the bony lamellae, which are arranged in concentric layers. 



In a transverse section of the diaphysis of a long bone, it is noticed that the 

 fundamental substance is excavated by an infinite number of vascular canaliculi, 

 named Haversian canals (Figs. 9, 10). These canals measure from l-2500th to 

 l-200th of an inch in diameter, and are parallel to each other and to the larger 



Fig. 9. 



VERTICAL SECTION OF BONE, 



showing the network of 

 Haversian canals. 



