THE OSSEOUS TISSUE. 167 



Gutters or channels corresponding to blood vessels. 



5. Cavities subservient to transmission. 



Notches , when superficial and formed in the edges of 

 bones. 



Foramina, when they pass through or perforate the bone. 



Canals, when their passage is of great extent, as the 

 vertebral or medullary canal. 



Clefts or Fissures, if they are longitudinal and narrow. 



The structure of bone consists of several elements. 



By the naked eye the fibrous arrangement is observed, 

 and the fibres, as already stated, assume two forms, the one 

 forming the compact, the other the spongy or cancellated 

 structure. A modification of the spongy, in the medul- 

 lary cavity, receives the name of the reticular tissue. 



The compact tissue occupies the outer surface of the 

 bones, and has its fibres compressed so as to form a com- 

 pact, firm and dense tissue. 



Bone, treated with nitric acid, is made soft and its fibrous 

 character clearly shown. In the long bones the fibres ar- 

 range themselves longitudinally ; in the flat, they diverge 

 like radii from a certain point, while in the thick they are 

 very irregular. The osseous fibres are found to be lamin- 

 ated as well as fibrous, that is, consisting both of filaments 

 and plates or laminse ; and the intervals between them are 

 only seen with the microscope. These, however, become 

 gradually more and more distinct as they approach the 

 extremity of the bone, and are there continuous with the 

 cellular or reticulated tissue. Indeed, says Cruveilhier, 

 " the compact tissue is nothing more than an areolar sub- 

 stance, the meshes of which are extremely close and much 

 elongated," Diseases also frequently show the compact 

 tissue changed into the spongy, and, vice versa, the spongy 

 into the compact 



The spongy or honey-comb structure (Fig. 30) pre- 

 sents cells and areolae of variable size and shape, all of 

 which communicate, and consist of filaments and fine 

 laminae, crossing, uniting, and separating in every direc- 

 tion. These cells contain marrow, and hence are called 



