482 



OF NUTRITION. 



the bone also is traversed by canals, on which the name of Haversian has been 

 bestowed, after their discoverer ; these canals run for the most part in the direc- 

 tion of the laminae ; but they have many transverse communications, both with 

 each other and with the medullary cavity, so that they form a complete network, 

 which is lined by a continuation of the highly-vascular membrane of the 

 latter. When a thin transverse section of a long bone is made, and is highly 

 magnified, .it is seen that the bony matter of the greater part of its thickness is 

 arranged in concentric circles round the orifices of the canals ; these circles are 

 marked by a series of stellated points ; and when the latter are magnified still 

 more highly, it is seen that they are cavities of a peculiar form, which seems 

 characteristic of Bone. They are usually oval or lenticular in form ; and are 

 so placed that one of their largest surfaces is turned from, and the other to- 

 wards, the Haversian canal. Their long diameter is commonly from l-2400th 

 to l-1600th of an inch ; their short diameter about one-third, and their thick- 

 ness about one-sixth of their length. From all parts of these cavitiesf but 

 especially from their two largest surfaces, proceed a large number of minute 

 tubuli, which traverse the substance of the bone, and communicate irregularly 

 with one another. Their direction, however, possesses a certain degree of 



[Fig. 114. 



[Fig. 115. 



Transverse section of the compact tissue Transverse section of the compact tissue of a Tibia 



of a long Bone; showing 1, the periosteal from an aged subject, treated with acid; showing the 



layer; 2, the medullary layer, and the inter- appearance of lamellae surrounding the Haversian ca- 



mediate Haversian systems of lamellae, each nals. Portions of several systems of lamella; are seen, 



perforated by an Haversian canal. Mag- The appearance of the lacunae, when their pores are 



nified about 15 diameters.] filled with fluid, is also seen, as well as the radiation from 



the canals which then remains. From Mr. Tomes.] 



determinateness ; for those passing off from the inner surface converge towards 

 the Haversian canal ; whilst those passing off from the surface diverge in the 

 contrary direction, so as to meet and inosculate with those proceeding inwards 

 from the cavities of the next annulus. In this manner, a communication is 

 kept up between the Haversian canal, and the most external of its concentric 

 lamellae of bone, or between the great medullary cavity, and the most distant 

 of the lamellae which are concentric with it. It is not to be imagined, how- 

 ever, that blood can be conveyed by these tubuli, they: size being far too small ; 

 the diameter of each canal, at its largest part, is estimated at from l-14,000th 

 to 1 -20,000th of an inch, whilst that of the smaller branches is from l-40,000th 

 to l-60,000th of an inch. It may be surmised, however, that they may convey 

 matter poured forth by the blood-vessels, which may serve for the nutrition of 

 the bone-substance. We know that, in young animals, this substance is con^ 

 tinually undergoing increase and renewal ; and even in adults some interstitial 



