268 



OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



Fig. 60. 



they have many transverse communications, both with each other and with the 

 medullary cavity, so that they form a complete net- 

 work (Fig. 60), which is lined by a continuation of 

 the membrane of the latter. Their diameter varies 

 from l-200th to l-2000th of an inch ; the average 

 being probably about l-500th. The smaller ones 

 contain only a single capillary vessel ; but several 

 such vessels seem to exist in the larger ones, together 

 with adipose matter. When a thin transverse sec- 

 tion of a long bone is made, and is highly magnified, 

 it is seen that the bony matter of the greater part of 

 its thickness is arranged in concentric circles round 

 the orifices of the canals (Fig. 61) ; these circles are 

 marked by a series of stellated points ; and when the 

 latter are magnified still more highly (Fig. 62), they 

 are seen to be cavities or lacunas of a peculiar form, 

 which seems characteristic of Bone. They are usu- 

 ally oval or lenticular in form ; and are so placed, 

 that one of their largest surfaces is turned from, and 

 the other toivctrds, the Haversian canal. Their long 

 diameter in Man is commonly from l-2400th to 

 l-1600th of an inch ; their short diameter is about 

 one-third, and their thickness about one-sixth, of 

 their length. It has been shown by Mr. J. Quekett, 1 

 that there are differences in the form and size of the 

 lacunae, in the several classes of Vertebrated ani- 

 mals, sufficiently characteristic to allow of the as- 

 signment of minute fragments of bone, with the 



aid of the microscope, to their proper group. 



Fig. 61. 



Haversian canals, seen on a lon- 

 gitudinal section of the compact 

 tissue of the shaft of one of the 

 long bones ; 1, arterial canal ; 2, 

 venous canal ; 3, dilatation of an- 

 other venous canal. 



Portion of transverse Section of Human Clavicle, showing the orifices of the Haversian canals, and the concen. 

 trie arrangement of the laminae of bony matter, and of the lacunae around them. Magnified 85 diameters. 



1 " Transactions of the Microscopical Society," vol. ii. See also "Princ. of Pliys., Gen. 

 and Comp.," \ 202, Am. Ed. 



