STRUCTURE OF OSSEOUS TISSUE. 119 



physes of the long bones, the osseous tissue forms a thin layer 

 of compact substance on the surface ; but in the interior small 

 laminae exist, inclined at various angles to one another, between 

 which are medullary spaces containing vessels and connec- 

 tive tissue with marrow and fat cells. The substance of the 

 bone consequently here presents a spongy character. In the 

 flat bones, tables of compact substance, corresponding to the 

 two principal surfaces, are superficially placed, between which 

 the osseous substance presents the same spongy character. 

 The compact bony substance is strongest in the diaphyses of the 

 long bones ; but even here, in the more internal parts which 

 surround the great medullary cavity, it presents the spongy 

 character which is more conspicuous in proportion as the 

 epiphyses are approximated. 



On making fine sections of the compact substance of the 

 tubular bones after removal of the mineral matter, some of the 

 finer characters may be very distinctly brought into view. 

 Sections carried perpendicularly to the long axis of the bone 

 exhibit larger or smaller round or slightly oval spaces, which 

 are seldom elongated in a longitudinal direction, but are often 

 bounded by slightly sinuous lines, and represent the transverse 

 sections of the Haversian canals hereafter to be described. 

 Around these the matrix of the bone forms concentrically 

 arranged ribbon-like striae, which, in a certain focus of the 

 microscope, in the portion nearest to the canals, appear radially 

 striated and somewhat darker than elsewhere. The number 

 of laminae succeeding one another from within outwards varies, 

 but the smaller canals have fewer than the larger. As many 

 as fifteen have been counted. The system of rings surrounding 

 each space is enclosed by similar rings pursuing a course 

 parallel to the external surface to the bone, so that the latter 

 may be differentiated from the former as being of a higher 

 order ; but since the systems of the first order cease, in some 

 instances nearer, and in others at a point more distant from 

 the surface of the bone, the number of the rings running con- 

 centrically with the general circumference of the bone is not 

 constant, but is smaller where the rings of the first order 

 approximate more closely to the surface. Only those which 

 course around the most superficial systems of the first order 



