THE STERNUM. 161 



stronf ligaments, which, on the external surface, have a par- 

 ticular radiated texture. The pits, at the upper part of the 

 sternum, are at the greatest distance one from another, and 

 as they descend, are nearer; so that the two lowest are con- 

 tiguous. 



The substance of the breast bone is cellular, with a very thin 

 external plate, especially on its internal surface, where we may 

 frequently observe a cartilaginous crust spread over it. On 

 both surfaces, however, a strong ligamentous membrane is 

 closely braced; and the cells of this bone are so small, that a 

 considerable quantity of osseous fibres must be employed in 

 the composition of it. Whence, with the defence which the 

 muscles give it, and the movable support it has from the 

 cartilages, it is sufficiently secured from being broken : for it is 

 strong by its quantity of bone ; its parts are kept together by 

 ligaments ; and it yields enough to elude considerably any 

 violence offered. 



The three pieces which compose this bone are very different 

 from each other. 



The^rs^ piece resembles a triangle, with the corners cut off. 

 The upper edge of it is thick, and has a regular depression in 

 the middle, to accommodate the trachea. On each side of this 

 depression is a superficial cavity, which, on viewing it trans- 

 versely, from before backwards, appears a little convex. Into 

 these cavities the ends of the clavicles are received. Imme- 

 diately below them, the sides of this bone become thinner; and 

 in each a superficial cavity, or a rough surface is to be seen, 

 where the first ribs are received or joined to the sternum. In 

 the side of the under end of this first bone, the half of the pit 

 for the second rib on each side is formed. The upper part of 

 the surface behind is covered with a strong ligament, which 

 secures the clavicles ; and is afterwards to be more particularly 

 taken notice of. 



The second, or middle division of this bone, is much longer, 

 narrower, and thinner, than the first; but, excepting that it is 

 a little narrower above than below, it is nearly uniform in its 

 14* 



