364 GENERAL ANATOMY. 



titude of small holes or orifices of canals into which very small 

 vessels penetrate. 



581. The density of the osseous tissue is very great, but 

 it is not the sam,e in all parts of the same bone. With refer- 

 ence to this circumstance, the substance of the bone is distin- 

 guished into compact and spongy or areolar. The first is cor- 

 tical, or situated at the exterior of the bones. The other is in- 

 ternal. 



The compact substance is that whose density is such that no 

 interstices are perceived in it by the naked eye, although it is 

 perforated with many medullary and vascular canals visible 

 to the microscope. In the long bones, these canals are longi- 

 tudinal. They have frequent lateral communications with the 

 great medullary canal, and the outer surface of the bone. 

 They are smaller towards that surface than towards the other. 

 Their mean diameter is the twentieth of a line. 



The areolar, or spongy substance, is that which forms small 

 cavities, distinct!)' visible to the naked eye. This substance 

 presents several varieties, of which the principal are the fol- 

 lowing: It consists of filaments more or less fine, and of la- 

 minae of a like tenuity, in the extremities of the long bones, 

 and in the substance of the short bones; of reticulated filaments 

 and laminae at the internal surface of the medullary canal of 

 the long bones; and of strong laminae, forming narrow areolae 

 in the broad and thin bones, especially in those of the skull. 



The two substances, or varieties of the more or less dense 

 tissue of the bones, are arranged in a particular manner in each 

 kind of bone. 



In the long bones, the body is formed of compact substance, 

 and the inner surface of the canal is bristled with some reticu- 

 lated filaments and laminae. Towards the extremities, the 

 compact substance greatly diminishes in thickness, the areolar 

 or spongy substance becomes more and more abundant and 

 fine, the great canal ends by becoming continuous with the 

 spongy substance, with which the whole extremity of the 

 bone is filled. 



In the broad bones, the two surfaces are formed of compact 

 substance. Wherever the bone is thin, these two laminae touch 



