OSSEOUS TISSUE. 



325 



Haversian canals, seen on a 



the lon s bones - * Arterial ca- 



_. , , . . . . _ _ nal. 2. Venous canal. 3. Dila- 



In both these positions, indeed, very tation of am)ther Ven0 u8 canal. 



scapula, articular portion of the ilium, &c.). Fi s- 214 - 



Less frequently, however, they are parallel 

 to each other as in the sternum. 



In the short, bones there is usually one 

 predominant direction for the Haversian 

 canals ; they being vertical in the vertebrae, 

 and parallel to the long axis of the extre- 

 mity, in the carpal and tarsal bones. In the 

 spinous processes, however, as in the cora- 

 coid and styloid processes, the canals are 

 arranged as in the shorter cylindrical bones. 



Finally, a few Haversian canals exist in 

 the walls of the cancelli of bone, but only 

 when they are of considerable thickness. 



Since the Haversian canals contain ves- 

 sels, they open first, externally on the outer 

 surface of the bone; and, secondly, internally 

 on the walls of the medullary canals and 

 spaces 



many of them can be seen by the unassisted 



eye; they being more numerous in proportion as the compact sub- 

 stance of the bone is thicker. The larger passages are, however, 

 merely for the vascular branches communicating with the proper 

 capillary plexus in the interior of the bone-substance. Kolliker 

 has never noticed coecal terminations of these vascular canals. The 

 internal network must, however, in some parts have little or no 

 communication with vessels from the surface of the bone as at the 

 points of insertion of many tendons and ligaments, &c. 



It should be added here that Tomes and De Morgan have de- 

 scribed still another kind of cavities in bone, which they have 

 named Haversian spaces. These are apparently formed by the ab- 

 sorption of previously-formed bone, between and parallel to the 

 Haversian canals; present rough parietes, and are sometimes formed 

 at the expense of portions of two or three of the Haversian sys- 

 tems or rods at the same time. After attaining their full diameter, 

 ossification commences within them, and each of them becomes a 

 new Haversian canal, surrounded by its concentric laminae, and its 

 lacunas and pores. These cavities are very large and numerous in 

 newly-formed bone situated near ossifying cartilage; they are, how- 

 ever, never absent in the oldest subject, and may be accepted as a 



