CHAP. V.] 



VESSELS OF BONE. 



107 



Fig. 19. 



pretty uniformly through the tissue, and inosculate everywhere with 

 one another. In the long and short bones they follow the same 

 general direction as the axis of thebone, and 

 are joined at intervals by cross branches. 

 The meshes thus formed are more or less 

 oblong (fig. 19). The deeper ones open 

 into the contiguous cancelli, with the ca- 

 vities of which they are continuous. 



The arteries and veins of bone usually 

 occupy distinct Haversian canals. Of 

 these the venous are the larger, and com- 

 monly present at irregular intervals, and 

 especially where two or more branches 

 meet, pouch-like dilatations, calculated to 

 serve as reservoirs for the blood, and to 

 delay its escape from the tissue. In many 

 of the large bones, particularly in the flat 

 and irregular ones, the veins are exceed- 

 ingly capacious, and occupy a series of 

 tortuous canals of remarkable size and very 

 characteristic appearance. These are well 

 described by Breschet in his elaborate e - Dilatation of another venous canal - 

 work on the venous system ; from which the accompanying figure 

 (fig. 20) is taken. These 

 canals run, for the most part, 

 in the cancellated structure 

 of the bones, and are lined 

 by a more or less complete 

 layer of compact tissue, whicl i 

 itself often contains minute 

 Haversian canals. The veins 

 they contain discharge them- 

 selves separately on the sur- 

 face. 



The Haversian canals vary 

 in diameter from -^^ to 



* ne S-ol) Of an inch, Or more. Venous canals in the diploe of the cranium. - After 

 1 * Breschet. 



the average being about ^L_. 



Their ordinary distance from one another is about T ^ of an inch. 

 They may be regarded as involutions of the surface of the bone, 

 for the purpose of allowing vessels to come into contact with it 

 in greater abundance. It is evident that the cancelli, and even the 



Firj. 20. 



