OSSEOUS TISSUE. 9 



the latter are larger, but do not weigh as much in proportion, 

 because the shell, or outer layer, is more expanded and thinner, 

 affording greater surface for muscular attachments. Whereas, 

 in the thorough-bred, a greater density of bone is necessary to 

 withstand thg intense concussion of speedy action ; therefore the 

 bones are increased in thickness of shell, affording greater strength 

 without apparent increase in size. 



OSSEOUS TISSUE. 



Ill bone tissue there are two modifications of texture, the com- 

 'pact and the cancellated. The former — hard, dense, and ivory- 

 like — is always situated externally; the latter — porous and 

 spongy — lies within. 



Although the compact 

 tissue appears uniformly 

 dense, and destitute of por- 

 osity, yet, if we transversely 

 sectate the shaft of a long 

 bone, and examine it under 

 the microscope, by trans- 

 mitted light, it is found to 

 contain numerous round 

 openings. These arc called 

 Haversian canals. They j-ig. 2. 



transmit blood-vessels, and Transverse section of compact osseous tissue. Au 



1 'J- J' 1 Haversian canal, 3, is seen surrounded by concentric 



run m a longitudinal or j^y^.^^ forming an Haversian cylinder, l'. The dark 



slio'htlv obliciue direction objects between the lamelUie are lacunae; 2 is a similar 



^ . f ' cylinder, showing also the canaliculi ; 4 4, Portions of 



opening on either the outer the peripheral and interstitial lamellK. 



or inner surface of the 



bone ; they also have many transverse branches of communica- 

 tion, which are often of greater diameter than the trunks. These 

 canals vary from about ir^yth to TTrooth of an inch in diameter, 

 and surrounded by concentric layers or lamellae of bone, which 

 are shady or opaque, and vary in number from two or three to six, 

 the internal being most distinct, each succeeding one becoming less 

 so. Between them may be seen many dark, irregularly-oval 

 objects — the minute reservoirs or lacuna], containing the hone- 

 corpuscles, from which numerous radiating processes occupy the 

 canaliculi ; the latter are minute canals, which establish a com- 

 munication between the Haversian canals and the lacunae. On 

 the external part of the shell of the compact tissue of long bones 



