48 ELEMENTARY TISSUES. 



tioned, 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 occupied by a multitude of little spaces, 

 called lacunae, with very minute canals or canaliculi, as they 

 are termed, leading from them, and anastomosing with similar 

 little prolongations from other lacunae (Fig. 16). In very thin 

 layers of bone, no other canals than these may be visible ; but 

 on making a transverse section of the compact tissue, e. g., of 

 a long bone, as the humerus or ulna, the arrangement shown 

 in Fig. 16 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 concentric 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 ex- 

 tremities of small canals which run lengthwise through the 

 bone (Fig. 17), and are called Haversian canals, after the 

 name of the physician, Cloptou Havers, who first accurately 

 described them. 



The Haversian canals, the average diameter of which is 

 of an inch, contain bloodvessels, and by means of them blo 

 is conveyed to all, even the densest parts of the bone ; the mi- 

 nute canaliculi and lacunae absorbing nutrient matter from the 

 Haversian bloodvessels, and conveying it still more intimately 

 to the very substance of the bone which they traverse. The 

 bloodvessels enter the Haversian canals both from without, by 

 traversing the small holes which exist on the surface of all 

 bones beneath the periosteum, and from within by means of 

 small channels which extend from the medullary cavity, or 

 from the cancellous tissue. According to Todd and Bowman, 

 the arteries and veins usually occupy separate canals, and the 

 veins, which are the larger, often present, at irregular intervals, 

 small pouch-like dilatations (Fig. 17). 



The lacunce are occupied by nucleated cells, or, as Dr. Beale 

 expresses it, minute portions of protoplasm or germinal matter; 

 and there is every reason to believe that the lacunar cells are 

 homologous with the corpuscles of the connective tissue, each 

 little particle of protoplasm ministering to the nutrition of the 

 bone immediately surrounding it, and one lacunar particle 

 communicating with another, and with its surrounding district, 

 and with the bloodvessels of the Haversian canals, by means 

 of the minute streams of fluid nutrient matter which occupy 

 the canaliculi. 



Besides the concentric lamellce of bone-tissue which surround 

 the Haversian canal in the shaft of a long bone, are others, es- 



