30 ANATOMY AND PHYSIOLOGY. 



brane, in which is contained the medulla or marrow. The medulla 

 is in young persons of a bloody nature, in old persons it is oily. 

 The canal serves to make the bone much stronger than if it were 

 solid with the same quantity of material ; on a principle which is 

 well understood by machine makers, who often make use of hollow 

 pillars and shafts, to gain additional strength without additional 

 expense. The marrow is put into the canals, not to oil the bones, 

 as many people erroneously suppose, but because there is no empty 

 space permitted in the body, and fatty matter is the lightest that 

 could be used for filling them. Besides, the marrow serves the 

 same purpose as the fat in other parts of the body ; it is a store of 

 nourishment whence the body can be supported when unable to 

 take any nourishment from without. In fevers, for instance, where 

 the patient scarcely tastes food for perhaps three weeks, he is main- 

 tained on the superfluous parts of his own body, and hence the 

 sunken cheeks and shrunk shanks of such a sufferer when begin- 

 ning to recover. That this marrow is not of any use to the bone 

 itself is sufficiently proved by the fact, that in birds there is none, 

 but the bones are very thin and their canals large in order to be 

 light, and instead of marrow, they are filled with air. There is a 

 common notion that the marrow is exceedingly sensible, and per- 

 sons remark how painful the application of a saw must be in an 

 amputation, from its tearing through the marrow. Now the fact is, 

 that the marrow is very little if at all sensible, and all the pain felt 

 in sawing the bone is a sort of jarring communicated to the soft parts 

 which have been already divided. 



The irregular bones, resembling in shape two or more of the 

 preceding orders, have some of their parts resembling the round 

 bones, and others resembling the long and flat. 



Where the bones touch one another, they are particularly smooth, 

 and their surfaces are adapted to one another. Besides, to obviate 

 friction, they are covered at these places with cartilage or gristle. 

 Cartilage is intermediate in hardness to bone and what are properly 

 called the soft parts, it is firm and resisting, and yet it has a great 

 degree of elasticity. In some parts of the body we have cartilages 

 serving for continuations to bones, such as those which continue 

 the ribs, and connect them to the breast-bone, and they are exactly 

 similar to bones from which the earthy part has been dissolved out 

 by an acid. But the cartilaginous crusts which cover the articular 

 ends of bones are of a very beautiful and peculiar structure. When 

 a portion of bone with its cartilage has been macerated in water for 



