CHAP. V.] DEVELOPMENT OF BONE. 115 



respects, to every other example of the compact tissue ; the chief 

 difference consisting in the direction taken by the Haversian canals, 

 which is irregular where the tissue follows an irregular course. In 

 general, however, the canals, with the Haversian rods forming their 

 sheaths, run in the direction in which the tissue needs the greatest 

 strength. Thus, in the long bones it is vertical ; and in those flat 

 bones, which have to support weight, it is also more or less vertical; 

 while in those designed to sustain the action of forces of other 

 kinds it is liable to corresponding variety. 



So beautifully mechanical is this disposition of the Haversian 

 systems in the compact tissue, that we need not smile at the de- 

 scriptions of Gagliardi, who, with imperfect means of observation, 

 appears to have been at least faithful in his attempts to delineate 

 nature. The periosteal and medullary layers are true plates of 

 bone, and the Haversian systems are true fibres or pins, all con- 

 nected with one another by direct continuity of tissue, and most 

 artfully arranged for the mechanical ends in view : and we cannot 

 sufficiently admire the skill which has caused the means, employed 

 for these ends, to conspire with those which were indispensable for 

 the due nutrition of the tissue. 



In the ordinary cancellated texture, each cancellus must be 

 regarded as a little medullary cavity, containing, as it does, 

 medulla and highly vascular medullary membrane. The plates of 

 bone which form its walls consist of lamellae, among which lacunae, 

 with their pores, are scattered ; and they sometimes, when thick, 

 contain Haversian canals. Usually, however, the pores of these 

 laminae communicate directly with the cavity of the cancellus to 

 which they belong. 



Nerves of Bone. The skill of anatomists has hitherto failed to 

 demonstrate the presence of nerves in the interior of bones. Some 

 nerves pass through bones, but no supply strictly to the osseous 

 matter has yet been proved. Yet there is little doubt that the 

 vascular surface of bones is furnished with nerves; the painful- 

 ness of many affections of the periosteum, and of the medullary 

 membrane, seems to place this beyond dispute. 



Development. In the earliest period at which the skeleton can 

 be detected among the other tissues of the embryo, it is found to 

 consist only of a congeries of cells, constituting the simplest form 

 of cartilage. These increase in number and in density, and become 

 surrounded and held together by an intercellular substance in small 

 quantity ; thus forming the temporary cartilage, which subsequently 

 becomes converted into bone. The temporary cartilages have the 



i 2 



