CONNECTIVE TISSUE. 225 



made up of cancellous tissue, with only a thin layer of compact 

 bone at their peripheries. Flat bones exhibit the cancellous 

 structure in the middle portion, the diploe of skull-bones, 

 while the layers on the outer and inner surface are formed 

 entirely of compact bone, the outer layer being generally 

 broader and richer in blood-vessels than the inner. Thin, small, 

 flat bones, such as the ethmoideal, turbinate, etc., may be con- 

 sidered as flattened trabecula3 of the cancellous variety. The 

 shaft-bones have a broad investment of compact structure in 

 their middle, i. e., diaphyseal portion, while this tissue gradually 

 decreases toward the ends, the epiphyses. These parts, as well 

 as the large central marrow space, exhibit the cancellous struct- 

 ure in greatly varying amounts. H. Meyer first drew atten- 

 tion to the fact that the trabeculae of the cancellous structure, 

 especially in the epiphyseal extremities of shaft-bones, were built 

 up according to a certain law, and J. Wolff, with the assistance 

 of Culmann, has explained, according to mathematical principles, 

 the regular arrangement of the trabeculae in the directions of 

 lines of pressure and traction. 



(b) Cortical or Compact Bone-tissue is composed of parallel 

 lamellae, closely packed together in intimate relation with the 

 blood-vessels. The cortex of both flat and shaft bones consists of 

 a concentric system of lamellae surrounding the central marrow 

 space, and this concentric system in its middle portion is trav- 

 ersed, usually at right angles, by a number of systems of lamellae, 

 each surrounding a central blood-vessel. Thus, two peripheral 

 systems of lamellae originate, of which the outer is always the 

 broader, and well marked, the inner often but little developed. 



The middle portion, lying between the two peripheral sys- 

 tems, is traversed more or less rectangularly by the Haversian 

 systems. In transverse sections of the cortex of shaft-bones, the 

 lamellae of the two peripheral systems run longitudinally, while 

 the Haversian systems are cut transversely or obliquely. Be- 

 tween the latter there are the longitudinal lamellae of the so-called 

 " interstitial or intermediate " bone-tissue. (See Fig. 87.) 



The Haversian system is composed of lamellae which are dis- 

 posed in concentric layers around capillary blood-vessels. Such 

 systems of lamellae are regularly arranged in the cortex of the 

 long bones, and irregularly in the cortex of flat bones. The outer 

 contour of a system is never smooth and even, but composed of 

 a number of shallow protrusions, viz., the first-formed territories, 

 and the aggregation of these formations gives the outer contour 

 15 



