96 The Vertebrate Organ Systems 



is a complex material containing many marrow cells, fat cells, other 

 types of cells, blood vessels, and nerves. The entire bone is sheathed in 

 a tough, connective tissue material known as the periosteum. Penetrat- 

 ing the compact tissue of the bone are many small openings through 

 which blood vessels and nerves pass into the bony tissue and marrow. 



/SYNOVIAL FLUID 



''ARTICULAR CARTILAGE PERIOSTEUM- 



iMARROW 



vCOMPACT BONE 



jblood vessel 



«MARROW CAVITY 



'ARTICULAR UCAMENT 



Fig. 27. — Diagram of the structure of a long bone and joint. (From Neal and Rand: 

 Chordate Anatomy, The Blakiston Company.) 



Microscopically a cross section through the compact bone (Fig. 

 28) shows that it consists of many series of concentric rings. In the 

 center of each series of these is a canal, the Haversian canal. Each 

 series of rings or lamellae, with its central canal, is known as a Haver- 

 sian system. Located more or less between these rings are small spaces, 

 the lacunae, which contain the living bone cells, the osteoblasts. Ex- 

 tending outward from each lacuna are numerous tiny passages, canali- 

 cules. These form passageways for the movements of fluids from the 

 Haversian canal into the lacunae, from one lacuna to another, and from 

 one lamella to another. These canalicules do not communicate from one 

 Haversian system to another as the systems are effectively set off 

 from each other by special cementing lines. 



Through each Haversian canal there pass usually two blood vessels. 

 These blood vessels are nearly always capillaries, and the fluids carried 

 by them are able to pass into the communicating canalicules and thence 

 to the lacunae. 



The blood vessels are able to pass into the bony tissue through 

 special canals which communicate with both the Haversian canals and 

 the marrow cavity. Frequently the Haversian canals also communi- 



