44 HAND-BOOK OF PHYSIOLOGY. 



branches from the periosteal vessels entering the little foramina on the 

 surface of the bone, and finding their way to the Haversian canals, to be 

 immediately described. The long bones are supplied also by a proper 

 nutrient artery which, entering at some part of the shaft so as to reach 

 the medullary canal, breaks up into branches for the supply of the mar- 

 row, from which again small vessels are distributed to the interior of the 

 bone. Other small blood-vessels pierce the articular extremities for the 

 supply of the cancellous tissue. 



Microscopic Structure of Bone. Notwithstanding the differences 

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

 the microscope to be essentially the same. 



FIG. 45. 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 extend- 

 ing 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 lamella? are visible. X 150. (Sharpey.) 



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

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

 called lacuncB, with very minute canals or canaliculij as they are termed, 

 leading from them, and anastomosing Avith similar little prolongations 

 from other lacunae (Fig. 45). 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. 45 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 lacunce and canaliculi following the same con- 

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

 "which, indeed, they communicate. 



