4:6 HANDBOOK OF PHYSIOLOGY. 



arrangement just mentioned, the structure of all bone is found under 

 the microscope to be essentially the same. 



Examined with a rather high power, its substance is found to contain 

 a multitude of small irregular spaces, approximately fusiform in shape, 

 called lacunce, with very minute canals or canaliculi, as they are termed, 

 leading from them, and anastomosing with similar little prolongations 

 from other Iacuns8 (Fig. 52). In very thin layers of bone, no other canals 

 than these may be visible; but on making a transverse section of the 

 compact tissue as of a long bone, e. g., the humerus or ulna, the arrange- 

 ment shown in Fig. 52, can be seen. 



The bone seems mapped out into small circular districts, at or about 

 the centre of each of which is a hole, and around this an appearance as 

 of concentric layers the lacuna and canaliculi following the same con- 



FIG. 52. Transverse section of compact bony tissue (of humerus \ Three of the Haversian 

 canals are seen, with their concentric rings ; also the corpuscles or lacunae, with the canaliculi ex- 

 tending from them across the direction of the lamellae. The Haversian apertures had got filled 

 with debris in grinding down the section, and therefore appear black in the figure, which represents 

 the object as viewed with transmitted light. The Haversian systems are so closely packed in this 

 section, that scarcely any interstitial lamellae are visible. X 150. (Sharpey.) 



centric plan of distribution around the small hole in the centre, with 

 which, indeed, they communicate. 



On making a longitudinal section, the central holes are found to be 

 simply the cut extremities of small canals which run lengthwise through 

 the bone, anastomosing with each other by lateral branches (Fig. 53), 

 and are called Haversian canals, after the name of the physician, Clop- 

 ton Havers, who first accurately described them. The Haversian canals, 

 the average diameter of which is -g-i-g- of an inch, contain blood-vessels, 

 and by means of them blood is conveyed to all, even the densest parts of 

 the bone; the minute canaliculi and lacunaB absorbing nutrient matter 



