198 



USE OF THE MARROW. 



pact texture ; while the centre is occupied by a substance 

 of more soft and spongy nature. The proportions of 

 these two^arts vary in different bones, and in different 

 portions of the same bone. Thus, the compact substance 

 composes chiefly the shaft or body of the bone ; while 

 the head and the other extremity of the bone is formed 

 almost wholly of the spongy substance surrounded by a 

 thin covering of the compact. In the centre of the long 

 bones, a hollow space is left which is filled by a fatty 

 substance, called tne marrow. 



Emily. I connot comprehend what purpose is an- 

 swered by this spongy structure, which composes so 

 great a portion of the bone. To me it seems to increase 

 unnecessarily the size and weight of the bones, without 

 adding in the least to their strength. 



Dr. B. You are entirely mistaken, for the very 

 effect of this kind of structure is to increase the strength 

 of the bone. If the compact parts of the bone were all 

 united together into a solid piece without any space in 

 the centre, the bones being diminished in size, would 

 more easily yield to external force ; for it is demonstra- 

 ted by mathematics, that the same quantity of matter in 

 the form of a hollow cylinder is capable of resisting a 

 greater external force, than in the form of a solid cylin- 

 der, the length being the same in both. Hence, it is 

 generally found that in those parts of the body where 

 strength is required without much additional weight, the 

 bones have considerable spongy substance in their centre. 



JEmily. And the marrow what is the use of this ? 

 A store of nourishment laid up for the use of the bone ? 



Dr. B. It cannot be for the nourishment of the bone, 

 and yet it is difficult to say what its actual use is. It has 

 been supposed to be diffused through the substance of 

 the bone, and thus to render them less brittle. It has also 

 been conjectured that its use in the animal economy is 

 the same as that of other animal oils, and that the centre 

 of the bones being a very convenient reservoir, it was 

 placed here for this reason simply. 



