OF THE BONES. 363 



and vertebral canal, the pelvis, the thorax, the nasal fossae, the 

 orbits, &c. ; or even to form a hole or a conduit, as the spheno- 

 palatine hole, the foramen lacerum posterius, &c. the orbital, 

 palatal, and other conduits. 



Of these simple or compound cavities, some lodge organs, 

 others furnish insertions, and others serve to transmit, or afford 

 a passage to certain parts. 



In certain places of the bones, there occur a multitude of 

 small eminences and depressions, very close to each other. 

 This constitutes impressions or inequalities which serve for 

 insertions. 



579. The bones have internal and closed cavities, which 

 are called medullary cavities, because they contain the medul- 

 la, or fat of the bones.(169) 



The long bones have a large cylindrical medullary cavity, 

 which occupies their body, or middle part, and which, at its 

 extremities, communicates with the areola3 of the spongy sub- 

 stance. This cavity lodges the medullary system, and ren- 

 ders the bone lighter under the same volume, and stronger 

 with the same weight. 



The extremities of the long bones, the short bones, the broad 

 bones, and especially their thick edges, contain areolar cavi- 

 ties, which also lodge marrow. 



Lastly, there are some also whose substance is compact, con- 

 taining only microscopic medullary cavities. 



580. The bones have also vascular canals for the vessels of 

 the marrow, and for those of their proper substance. 



Each long bone has at least one canal of this kind, which 

 passes obliquely through the walls of the medullary cavity, 

 penetrates into it from above downwards in the humerus, the 

 tibia and the fibula, and from below upwards in the femur, the 

 radius and the ulna. This canal gives passage to the vessels 

 and nerves of the medullary membranes. 



The extremities of the same bone, the short and thick bones, 

 and the thick edges of the broad bones, are furnished with a 

 very great number of wide canals, which in like manner afford 

 passage to vessels, and especially to large veins. 



Lastly, the whole surface of the bone is riddled with a mul- 



