162 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



more minute passages, be excavated in its substance, it is evident that, even 

 though blood cannot circulate through it, a nutritive fluid drawn from the 

 blood, may be carried into its minutest parts. This is the kind of structure 

 which we find in Bone, and in the principal substance of Teeth. The mode 

 in which it is generated, will become the subject of inquiry hereafter. 



193. When examined with the naked eye, it is seen that Bone possesses 

 in some degree a laminated texture : in the long bones, the external and in- 



Fig. 61. 



Portion of Transverse Section of Human Clavicle, showing the orifices of the Haversian canals, and 

 the concentric arrangement of the laminse of bony matter, and of the lacunae, around them. Magnified 

 t-5 diameters. 



ternal laminae are arranged concentrically round the medullary canal ; and in 

 the flat bones, they are parallel to the surface. Towards the extremities of 

 the long bones, and between the external plates of the flat bones, are a num- 

 ber of cancelli, or small hollows bounded by very thin plates of bone ; these 

 communicate with the medullary canal where it exists ; having, like it, an 

 extremely vascular lining membrane; and their cavities being filled with a 

 peculiar adipose matter. The hard substance of the bone also is traversed by 

 canals, on which the name of Haversian has been bestowed, after their disco- 

 verer ; these canals run for the most part in the direction of the laminae ; but 

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

 the medullary cavity, so that they form a complete network, 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 1-500. 

 The smaller ones contain only a single capillary vessel ; but several such ves- 

 sels seem to exist in the larger ones, together with adipose matter. When a 

 thin transverse section 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 ; these circles are marked 

 by a series of stellated points ; and when the latter are magnified still more 

 highly, it is seen that they are cavities or lacunae of a peculiar form, which 

 seems characteristic of Bone. They are usually oval or lenticular in form ; 

 and are so placed, that one of their largest surfaces is turned from, and the 



