THE CONNECTIVE TISSUES. 71 



( "in;! Is, which contain the nutrient bloodvessels, and which anasto- 

 mose with each other throughout the whole substance of the tissue. 

 The nourishing lymph, derived from the blood, reaches the cells 

 through the canaliculi and lacunae, which connect with each other 

 to form a network of minute channels and spaces pervading the 

 bone, and not only opening into the Haversian canals, but also upon 

 the external and internal surfaces of the tissue. 



In the shafts of the long bones the Haversian canals lie for the 

 most part parallel with the axis of the bone, with short transverse 

 branches connecting them with each other. It is around these lon- 

 gitudinal Haversian canals that the laminse of bone are arranged 

 in concentric tubular layers. Each Haversian canal, with the 

 laminae surrounding it, is known as an Haversian system. Between 

 these Haversian systems there are excentric laminae of bone, which 

 do not conform to the concentric arrangement of the Haversian 

 systems. 



In the spongy or cancellated variety of bone the thin plates of 

 that tissue derive their nourishment from the lymph of the con- 

 tiguous marrow filling the spaces between them, and there is no 

 occasion for Haversian canals. The concentric arrangement of the 

 laminae is, therefore, absent, 



Except where bounded by cartilage at the joints, the external 

 surfaces of the bones are covered by a fibrous investment, the 

 periosteum, in which the bloodvessels supplying the bone ramify 

 and subdivide before sending their small twigs into the Haversian 

 canals of the compact bone. The deep surface of the periosteum 

 contains connective-tissue cells, " osteoblasts," capable of assuming 

 the functions of bone-corpuscles and producing bone. These facts 

 explain the importance of the periosteum for the nutrition and 

 growth of bone. The tendons and ligaments attached to the 

 bones merge with the periosteum, which has a similar fibrous struct- 

 ure and serves to connect them firmly with the surface of the 

 bone. 



The central cavities of the long bones and the spaces of cancel- 

 lated bone are occupied by marrow, which may be of two kinds, the 

 " red " or the " yellow." A description of the structure of marrow 

 must be deferred until the other varieties of the connective tissues 

 have been considered. 



In the embryo the parts which are destined to become bony first 

 consist of some other variety of connective tissue, either cartilage 



