BONE. 297 



modification, there being an insensible transition from the 

 one to the other. 



Drs. Todd and Bowman recognise two forms of Haversian 

 canals, one of which carries veins, the other arteries, a 

 single vessel being distributed to each; those canals which 

 contain the veins are stated to be larger than those which 

 convey the arteries, and to be dilated into a pouch or sinus 

 at the situation where two or more canals unite to form a 

 single larger tube. 



Lamellae. 



The more essential constituents of true and fully developed 

 bone are, as already observed, lamellae and bone cells; the 

 medullary cells and canals just described are merely definite 

 spaces existing between the lamellae, the arrangement and 

 ultimate structure of which we shall in the next place pro- 

 ceed to notice. 



It is principally by the successive development of new 

 lamellae that bones increase in diameter ; these are usually 

 deposited in the direction of the axis of the bone ; if, there- 

 fore, a transverse section of a long bone be made and examined 

 with the microscope, the lamellae will be seen to be arranged 

 as follows : first, several layers will be observed to pass entirely 

 round the bone, secondly, others will be noticed encircling 

 each Haversian canal, and lastly, irregular and incomplete 

 lamellae occupy the angular spaces intervening between the 

 sets of lamellae concentrically disposed around each canal. 

 (See Plate XXXII. Jigs. 1, 2, 3.) 



The number of layers which pass interruptedly around 

 the bone are not very numerous, being generally less than 

 twelve ; the amount of those which encircle each Haversian 

 canal varies from two or three to upwards of twelve, the 

 smallest number of lamellae usually appertaining to the 

 smallest canal. (See Plate XXXII. Jig. 1.) 



Examined with an object-glass of the fourth of an inch 

 focus the lamella, after the separation of its earthy matter 



B B 2 



