296 THE SOLIDS. 



partly occupied with medullary matter. They are also called 

 Haversian, after their discoverer. 



These canals are situated between the laminse of which 

 bone is composed, and take a course in the long bones, in 

 which they are best seen, parallel to their axes, being also 

 joined together by short transverse branches : they thus form 

 a network of tubes analogous to that exhibited by the 

 minute vessels which they convey and protect ; the form 

 and sizes of the meshes vary : in the long bones they are 

 elongated. (See Plate XXXII. fg. 4.) 



They communicate, in the long bones, with the medullary 

 cavity, dilating into cells or vesicles, from which a short tube 

 proceeds previous to their entrance into it ; they also open 

 upon the external surface of all bones by somewhat expanded 

 apertures, and inosculate freely with the medullary cells. 



Medullary canals are not all of equal diameter through- 

 out the compact tissue of a bone ; those situated between the 

 external plates are two or three times smaller than those 

 which are placed more internally. (See Plate XXXII. 



fig- 1.) 



In a transverse section the canals are seen to be either 

 circular, oval, or rarely angular. 



In the flat bones the course of the medullary canals is more 

 irregular than in the long bones ; in the parietal bones they 

 proceed diverging from the parietal protuberance towards 

 the margins of the bone ; and in the frontal from the supra 

 orbitar ridge towards the coronal suture. 



In the long bones, near their extremities and in the vicinity 

 of the articular cartilage, these canals end in blind or caecal 

 extremities, a single canal passing up into each of the promi- 

 nences, into a number of which the articular surface of bone 

 is elevated. 



It is these canals which impart the striated structure pre- 

 sented by bone, and which is visible to the unassisted eye ; 

 in longitudinal sections they are frequently cut through, and 

 their cavities exposed. 



The contents of medullary canals are similar to those of 

 the medullary cells, of which they are to be regarded as a 



