BOXE-MARRO W. 57 



kinds of systems Haversian, peripheral, peri-medullary, and 

 intermediate. 



A Havcr*ian system consists of a series of concentric 

 lamellae surrounding a Haversian canal, a small round canal 

 containing blood- and lymphatic-vessels and marrow, running 

 generally lengthwise the bone, and communicating and con- 

 tinuous with the interior marrow- cavity. Between the 

 adjacent lamellae at short intervals are spaces called lacunae, 

 occupied by the bone-cells; connecting the lacunae are numer- 

 ous minute passages called canaliculi, occupied by the processes 

 of the cells. The canaliculi afford communication for the 

 passage of nutrient fluids from the Haversian canals through- 

 out the adjacent lamellae. 



The peripheral or circumferential lamella? are a series of 

 subperiosteal lamellae, similar to the Haversian lamellae, 

 surrounding the entire bone. In places there are fibres 

 piercing the peripheral lamellae at right angles perforating 

 fibres, or fibres of Sharpey. 



The peri-medullary lamella? are a similar series surrounding 

 the marrow-cavity. 



The intermediate or interstitial lamella? are scattered irreg- 

 ularly about in the bone substance, and are the remains of old 

 systems that in the course of the growth of the bone have 

 been partially removed. 



The periosteum, the covering membrane of bone, is com- 

 posed of two layers: an outer firm fibrous protective layer; 

 and an inner loose layer well supplied with blood and connec- 

 tive-tissue cells (or osteoblasts), which has important functions 

 in the growth of bone and is hence called the osteogenetic 

 layer. 



Bone-marrow : The cavities in bone the cancelli of spongy 

 bone, the medullary or marrow-cavities of long bones, and 

 the Haversian canals contain a soft vascular tissue, the 

 ni<trroir. which is of two kinds, red and yellow. 



Red marrow occurs in cancellous bone in the adult, in all 

 bone in the young. It consists of delicate connective tissue, 

 a rich vascular network including large venous sinuses, 

 quantities of cells called marrow-cells, and fat-cells. The 

 marrow-cells are of several kinds, chief among which are : () 

 plastic rounded connective-tissue cells, many of \vhich in 



