36 AN INTRODUCTION TO ZOOLOGY 



off by a similar concentric arrangement of spaces, lacunae, in which 

 the bone cells or corpuscles are situated. A number of branching 

 canals, the canaliculi, come off from the lacunae and traverse the 

 lamellae, thus connecting together the rings of the lacunae and the 

 Haversian canal. The lacunae and canaliculi are also black in 

 sections, but in life are filled by cells which give off fine branches, 

 and in this way provide a series of channels through which the food 

 substances brought by the blood may be dispersed throughout the 

 bone. The whole arrangement of canal, lacunae and canaliculi is 

 known as an Haversian system. 



In addition to the Haversian, there is always a series of lamellae 

 on the outside of the bone and parallel with it. These are termed 

 the peripheral or circumferential lamellae. In the long bones the 

 marrow cavity is similarly lined by a series of perimedullary lamellae; 

 The small spaces between the three various sets are filled in by yet 

 other lamellae, the interstitial. 



The foregoing description of the appearance of bone applies 

 equally to both membrane and cartilage bone, the difference between 

 them being solely whether they are preceded by cartilage or not. 

 It is, however, based on the structure of the bones in the higher 

 vertebrates, such as the rabbit or man, and not strictly applicable 

 to the frog. In this animal we find that the Haversian systems are 

 never fully developed, so that in a transverse section of a long bone 

 we find only a continuous series of concentric lamellae, the outer 

 of which may be termed peripheral and the inner perimedullary. 



The free surface of bone is covered by a formative and nutritive 

 tissue, the periosteum, composed of two layers. The outer is fibrous 

 and plentifully supplied with blood-vessels, and the inner composed 

 of a layer of bone-forming cells or osteoblasts, left over from those 

 that produced the bone. 



The process of bone formation or ossification is complex, 

 and only its main features need be noted. In the case of a membrane 

 bone, the place it will take is occupied by connective tissue, whose 

 structure will be dealt with more fully later. Thin strands of the 

 matrix of the future bone are laid down, and the osteoblasts or 

 bone-formers arrange themselves in a more or less regular layer 

 around them. Here they deposit layers of bone, with the result 

 that bony spicules are produced. More strands of matrix are laid 

 down and surrounded by bone in a similar way, and so a network 

 of spicules is produced, at first loose but becoming more and more 

 compact. During this process certain osteoblasts are included in 

 the bone, and form the future bone corpuscles. In cartilage bone 

 a model of the bone is already present and enclosed in peri- 

 chondrium. Ossification takes place in this surrounding tissue in 



