44 ANATOMY FOR NURSES [Chap. IV 



in rings around canals, — Haversian canals, — which carry blood- 

 vessels in a longitudinal direction through the bones. Between the 

 lamellae are branciied cells which lie in cell-spaces, or cavities, called 

 lacunae (little lakes), and running out in a wheel-like or radial di- 

 rection from each lacuna are numerous tiny wavy canals called cana- 

 liculi, connecting one lacuna with another, and forming a system 

 of minute channels which communicate with each other and with 

 the Haversian canal. This constitutes an Haversian System, so 

 named from Havers, a celebrated anatomist. Many such systems 

 may be found in the shaft of a long bone. The spaces between 

 these systems are filled by lamellae arranged at irregular angles. 



Marrow. — Marrow consists of fi})rous tissue with blood-vessels, 

 fat-cells, marrow-cells and red corpuscles. There are two distinct 

 kinds of marrow, yellow and red. Yellow marrow contains a larger 

 per cent of fat, and is found in the medullary canals of the long 

 bones. Red marrow contains less fat, but is highly vascular and 

 occupies the spaces in cancellous bone. The function of marrow 

 is (1) to support the blood-vessels, lymphatics, and nerves; (2) to 

 ser^•e as a source of nourishment for bone ; and (3) as a location 

 for the formation of red corpuscles. (See page 143.) 



Periosteum. — All bones are covered, except at the joints, by a 

 vascular fibrous membrane, the periosteum (around the bone). It 

 consists of an outer fibrous layer and an inner vascular layer. The 

 attachment of the periosteum to bone is rendered firmer by inward 

 prolongations of the fibrous layer called the fibres of Sharpey. 



Blood-vessels. — Unlike cartilage, the bones are plentifully sup- 

 plied with l)lood. If we strip the periosteum from a fresh bone, 

 we see many bleeding points representing the canals (Volkman's) 

 through which the blood-vessels enter and where they leave the 

 bone. These blood-vessels proceed from the periosteum to join 

 the system of Haversian canals. Around the Haversian canals the 

 lamellae are disposed, while lying between them, arranged in circles, 

 are found the lacunae, which contain the bone-cells. Running from 

 one lacuna to another in a radial direction through the lamellae 

 towards the centre are the canaliculi. Following this scheme, it 

 will be seen that the innermost canaliculi run into the Haversian 

 canals, and thus is established a direct communication between the 

 blood in these canals and the cells in the lacunae connected with and 

 surrounding each Haversian canal. In this way the whole sub- 



