52 THE ESSENTIALS OF HISTOLOGY 



torn out of the deeper lamellae (fig. 61 c, c). Where tendons or liga- 

 ments are inserted into bone, their bundles of white fibres are prolonged 

 into the bone as perforating fibres. 



The lacunas are occupied by nucleated corpuscles, which send 

 branches along the canaliculi. 



The Haversian. canals contain one or two blood-capillaries and 

 nervous filaments, besides a little connective tissue ; and the larger 

 ones may also contain a few marrow-cells. There are also cleft-like 

 lymphatic spaces running parallel with the vessels and connected by 

 means of canaliculi with neighbouring lacunas. in the osseous substance 

 (fig. 62). 



FIG. 62. SECTION OF A HAVERSIAN CANAL, SHOWING ITS CONTENTS. 

 (Highly magnified.) 



a, small Arterial capillary vessel ; v, large venous capillary; n, pale nerve-fibres cut across; 

 I, cleft-like lymphatic vessel : one of the cells forming its wall communicates by fine 

 branches with the branches of a bone-corpuscle. The substance in which the vessels 

 run is connective tissue with ramified cells ; its finely granular appearance is probably due 

 to the cross-section of fine fibrils. The canal is surrounded by several concentric lamellae. 



The periosteum, which is best studied in sections from a bone which 

 has been decalcified in chromic or picric acid, is a fibrous membrane 

 composed of two layers, the inner of which contains many elastic 

 fibres. In the outer layer numerous blood-vessels ramify and send 

 from it branches to the Haversian canals of the bone. The periosteum 

 ministers to the nutrition of the bone, partly on account of the blood- 

 vessels it contains, partly, especially in young animals, on account of 

 the existence between it and the bone of a layer of osteoblasts or bone- 

 forming cells, a remainder of those which originally produced the bone. 



The marrow of bone is of a yellow colour in the shafts of the long 

 bones, and is there largely composed of adipose tissue, but in the can- 

 cellated tissue it is red, the colour being partly due to the large amount 

 of blood in its vessels. This red marrow is chiefly composed of round 

 nucleated cells -the marrow-cells (fig. 63, e-i) which resemble large 

 lymph-corpuscles, and, like these, are amoeboid. There are also to be 

 seen mingled with them a number of corpuscles somewhat smaller in 

 size, but nucleated and amoeboid, and of a reddish tint (fig. 63, j-i) ; 

 these are believed to be cells in process of development into coloured 



