88 



HUMAN ANATOMY. 



surface, and thus pin or bolt the layers together. These fibres are especially numer- 

 ous in the superficial lamellae beneath the periosteum, to which membrane they owe 

 their formation, and with which many seem to be directly continuous. They are 



Fig. ii6. 



Section of frontal bone, showing the absence of Haversian systems.  20. 



readily found on the surfaces of the lamellae of decalcified bone which have been 

 forcibly separated. Although usually consisting of bundles of fibrous tissue, it is 

 probable that in some cases the perforating fibres are elastic in nature. They are 

 sometimes imperfectly calcified and leave, therefore, on drying, tubular canals, 



which pierce the lamella from the ex- 

 FiG. 117. 



terior of the bone. Since the perfo- 

 rating fibres are associated genetically 

 with the periosteum, they are never 

 found in the secondary lamellae consti- 

 tuting the Haversian systems. 



The Haversian canals are con- 

 tinuations of the medullary cavity and 

 serve the important purpose of con- 

 veying the blood-vessels within the 

 compact substance ; from these vessels 

 the nutritive fluids pass into the peri- 

 vascular lymph-spaces between the 

 walls of the canal and the blood-ves- 

 sels and thence, by way of the cana- 

 liculi, which open into these lymph- 

 spaces, into the adjacent lacunae, and 

 so on into the surrounding portions of 

 the compact substance, the nutrition of which is thus maintained. Although the 

 average size of the canals is about .05 millimetre, those next the medullary cavity 

 are larger, some measuring . i millimetre or more in diameter, and contain, in addi- 



Lacuna cut 

 obliquely 



Canaliculi 



Haversian 

 canal 



Lacuna in 

 profile 



Portion of adjacent Haversian systems cut transversely. 

 X 250.. 



