STEUCTUEE OF BONE. 253 



of curious inquiry how they should be able to occupy exactly the places 

 and discharge with precision the functions of those which they are re- 

 placing? or, in the case of growth and development, why they should 

 combine so as to take on a determinate, and, as it were, predestined form ? 

 How is it that such a variety of structures spring up from the same orig- 

 inal cell ? How is it that the two halves of the body have such a sym- 

 metrical conformation in a majority of instances, the one being the exact 

 counterpart of the other, peculiarities which are often continued even 

 after the supervening of morbid conditions, as shown in such cases as 

 are known by the term of symmetrical diseases, in which a structural 

 change affecting one side of the body affects also the corresponding part 

 of the other side ? It appears to me that these and other such instances 

 of nutrition, growth^ and development can only be explained by admit- 

 ting, as a great and fundamental principle in physiology, that the primor- 

 dial germ being in all instances alike, its mode of development will de- 

 pend on the physical agents and conditions to which it is exposed : a 

 principle which, though it may seem of little moment at the first view, 

 carries with it consequences of the utmost importance at last. 



Second. Of the structure and development of bone. 



The skeleton in man is composed of 246 bones, which are usually di- 

 vided into three groups, the Ions:, flat, and irregular. Their 



. & , , & The skeleton. 



uses are purely mechanical, such as to give support to the 



soft parts ; to serve as levers on which the muscles, by their contractions, 



may act. 



In structure bone offers an imperfect division into the compact and 

 spongy. ' The compact is, however, a porous mass full of cells structure of 

 and passages. Through it there pass, more particularly in bone< 

 the longitudinal direction, canals for containing blood-vessels and nerves : 

 they are called haversian canals. These, which are well seen in a thin, 

 transverse section of bone as irregular circular openings, are surrounded 

 with lamellae, and in the basis substance occur hollow spaces, the lacuna?, 

 which, presenting a dark aspect, were formerly mistaken for solid corpus- 

 cles ; they are, however, cavities from which proceed minute channels or 

 canaliculi. In form the lacunas are irregularly oval ; the canaliculi of 

 those nearest to the haversian canal communicate directly with its cav- 

 ity, and there is so complete an inosculation between adjacent lacunas, 

 by means of these delicate tubes, that the whole so-called compact struc- 

 ture of the bone may be said to present a connected system of lacunas 

 and canaliculi. 



The diameter of these delicate channels of intercommunication is much 

 too small to permit the passage of blood-cells, yet through them the 

 plasma readily finds its way and thus carries forward the nutrition of 

 the entire bone. 



