32 CARTILAGE. BONE. TEETH. 



words, a Haversian canal. Very many of the canaliculi 

 from the nearest circle of bone-cells are also to be seen 

 running into the Haversian canal. In the long bones 

 the Haversian canals run parallel with the axis of the 

 shaft of the bone, and communicate with each other 

 at short intervals by transverse anastomotic branches 

 (PL VI. fig. I. 1, 2, 3). In the short and flat bones 

 they also pursue a determinate course, and anastomose 

 in a similar manner. These canals, which contain 

 the bloodvessels of the osseous tissue, tunnel its fun- 

 damental substance in all directions, terminating 

 either upon the external surface of the bone, or in its 

 medullary cavities. The nutrition of bone is effected 

 by means of the parts just described. The very 

 numerous orifices of the canaliculi in the walls of the 

 Haversian canal receive the nutritious fluid which 

 exudes through the walls of its contained bloodvessel, 

 and convey it throughout the network which they 

 and their parent bone-cells or lacunae form, to the 

 outermost of the series of concentric circles, 

 periosteum, The fibrous membrane which invests bone exter- 

 nally, called periosteum^ resembles perichondrium in 

 its structure ; it is an interlacement, or rather a felting, 

 of connective and elastic fibres, traversed by some 

 nerves and very numerous bloodvessels, and studded 

 with plasmatic cells which, as we shall see, play an 

 important part in the formation and growth of bone. 

 (PL VI. fig. V.) ' 



tie e s (hlll8iy e **~ ^ e me dullary cavities of bones are filled by mar- 

 row, which is in direct contact with their walls, for 

 the prevalent idea that the walls of these cavities are 

 lined by an internal periosteum, or medullary mem- 



