STRUCTURE OF BONE. 331 



ing to ordinary cellular texture ; and a bone, when 

 minutely examined, exhibits also the same appear- 

 ance of plates intermixed with fibres. In the outer 

 compact portion, indeed, the fibrous arrangement 

 of the particles is not so easily distinguished : but 

 it may be detected in young bones while they are 

 becoming ossified : and also in bones which have 

 been long exposed to the weather, or long mace- 

 rated in water. The interior of most bones, in the 

 higher classes of animals, presents distinctly the 

 appearance of irregular cavities, resulting from the 

 partial separation of the plates, and their mutual 

 crossings and fibrous connexions.* 



The different mechanical purposes for which 

 bones are employed in the animal economy require 

 them to be of different forms. Where a part is 

 intended to have compactness and strength, with a 

 very limited degree of motion, it is divided into 

 a great number of small pieces, united together 

 by ligaments ; and the separate bones are short 



* The great vascularity of bones is a remarkable feature in their 

 structure, Breschet has found that bones are traversed by numerous 

 veins of considerable size, which communicate largely together. The 

 existence of a multitude of very minute canals in the solid substance 

 of bones was discovered by Havers, about the end of the seventeenth 

 century. The form and arrangement of these canals have been in- 

 vestigated by Mr. Howship, who found that they compose a system 

 of nearly cylindrical cavities, passing in a longitudinal direction, 

 and freely communicating, by collateral branches, both with one 

 another, and also with the central medullary, or spongy cavities of 

 the bone. Their diameter is, on an average, about the 200th part of 

 an inch : but it varies from the 100th to the 400th. They are 

 every where lined with a vascular membrane of great tenuity, and 

 are tilled with an oily substance, or marrow. (Medico-Chirurgical 

 Trans, vi. 263; and vii. 387.) 



Purkinje has discovered in bones the constant presence of a great 



