NORMAL HISTOLOGY. 



FIG. 52. Lacunae and canaliculi in dried bone, cut par- 

 allel with the lamellae. X 300. 



periosteum, Sharpey's fibres are never found in the secondary lamellae 

 constituting the Haversian systems. 



The Haversian canals (.05-.! mm. in diameter) are continuations of 

 the medullary cavity and in the case of the larger ones, contain prolongations 



of the marrow-tissue. They serve 

 the important purpose of carrying 

 blood-vessels into the interior of 

 the compact bone. From these 

 vessels the nutritive fluids pass 

 into the canaliculi and the lacunae 

 and so on through the dense tis- 

 sue, the nutrition of the lamellae 

 and the enclosed bone-cells being 

 in this manner insured. The 

 individual canals are short and 

 communicate by oblique branches 

 with adjacent channels. They 

 also indirectly communicate with 

 the external surface of the bone by means of passages, the Volkmann canals, 

 within the circumferential lamellae. These canals open on the surface and 

 convey twigs from the periosteal blood-vessels into the lamellae other than 

 those of the Haversian systems. The twigs entering by the superficial canals 

 freely anastomose with those within the Haversian canals, the compact bone 

 being thus provided with a vascular network derived from both sources. 



The bone-cells are connective tissue elements imprisoned within the 

 lacunae, an arrangement similar in principle to that within the cornea where 

 the corneal cells lie within the lymph-spaces of the ground-substance. Sec- 

 tions of dried bone, useful as they are in affording striking pictures of general 

 arrangement, are entirely inadequate for the study of the bone-cells, since 

 the latter are shrunken and lost in the debris which, with air, fills the 

 lacunae in the ground specimens. In order to exhibit the bone-cells, after 

 fixation the tissue is decalcified, sectioned and stained and mounted in an 

 approved preserving medium. By such treatment the integrity of the cells 

 is insured, although the lacunae and canaliculi no longer show with diagram- 

 matic sharpness in consequence of being permeated with the mounting medi- 

 um instead of air. The bone-cells, after being stained in such decalcified 

 sections, appear as small lenticular or stellate 

 bodies within the lacunae, which they almost 

 or quite fill (Fig. 53). The deeply tinged nu- 

 cleus shows as a brilliant dot within the lighter 

 and faintly granular cytoplasm, which extends 

 from the stellate cell-body into the canaliculi 

 as delicate processes of variable length. 



The Periosteum. The external sur- 

 face of bones is closely invested, except when 

 covered with cartilage, by a fibrous mem- 

 brane, the periosteum, a structure of great 

 importance during development and growth, 

 and later for the nutrition and repair of the 

 osseous tissues. The adult periosteum consists of two layers, an outer fibrous 

 and an inner fibro -elastic ; during periods of growth, an additional 

 stratum, the osteogenetic layer, lies next and closely related with the exterior 

 of the bone. 





JK 







FIG. 53. Bone-cells lying within the 

 lacunae. X 530. 



