176 



HEALTH AND DISEASE 



STRUCTURE OF BONE 



When a long bone is cut through it is found to consist of a hard outer 

 whell of compact tissue enclosing a looser portion made up of thin bony 

 plates, interlacing with each otlier to form a number of spaces, and called 

 spongy or cancellated tissue. In the centre of this is a cavity {rnedullary 

 cavity) containing a soft reddish-yellow substance, the medulla or marrow. 

 The compact substance is thick in the shaft of the bone, but thin towards 

 the extremities, which are chiefly made ujj of cancellated structure. 



All bones are covered with a dense tousfh fibrous memljrane termed 

 jK'viosteum. It serves as a matrix in which the blood-vessels ramify and 



Fig. 281. — Transverse Section of Bone 



A A, Haversian Canals. The small irreg- 

 ular black spots are the Lacunae ; the lines 

 radiating from them are the Canaliculi. 



Fig. 282. — Longitudinal Section of Bone 

 A A, Haversian Canals. 



break up into smaller and smaller branches, prior to entering into the bone 

 tissue through small openings on its surface. A similar fibrous membrane, 

 though more delicate, also lines the interior of bones, and is known as 

 endosteum. This membrane is very thin, though rich in blood-vessels, 

 and affords nourishment to the inner portion of the bone and to the 

 marrow contained in it. Besides the vessels passing into the interior 

 from the periosteum, the long bones have also a nutrient artery, for which 

 a special opening is provided in the shaft of the bone called the medtdla7-y 

 foramen, and others less considerable situated around the extremities 

 {articular foramen). 



The intimate structure of l:)one can only be made out liy microscopic 

 examination under a power of 300 to 400 diameters. Although bone looks 



