164 THE OSSEOUS TISSUE. 



closely together and much condensed, or, on the other hand, 

 are more or less widely separated, and intersect each other in 

 every direction, so as to present the honeycomb appearance. 

 The first or condensed state forms the compact, the second, 

 the cellular or cancellated structure, of bone. The compact 

 gives strength and firmness to bone, and forms its external 

 osseous layer; while the cellular is delicate and spongy, 

 and designed to support the medullary membrane and its 

 marrow. 



The strength of bones varies equally with their density, 

 and that in different parts of the same bone. The thigh 

 bone, for instance, is not of equal strength in its whole 

 length; the body, having the compact tissue, is capable 

 of resisting a greater degree of force than the extremities, 

 which have only the light, spongy formation of delicate, 

 reticulated structure. All the long bones have a hollow 

 canal extending nearly their whole length, which is found 

 to add greatly to their strength, at the same time increas- 

 ing their lightness. This, Dr. Physick, in a very beautiful 

 and simple manner, illustrated by taking a sheet of paper 

 and rolling it into scrolls or hollow cylinders of various 

 diameters. He found the power of sustaining pressure to 

 increase, in a precise ratio with the increase of diameter, 

 up to a certain point. Now, taking a similar sheet and 

 rolling it into a solid cylinder, and comparing its strength 

 with that of the hollow one, it was found that its capacity 

 for sustaining weight, and its power of resisting pressure, 

 were much less. 



The doctor, by an equally simple experiment, shows that 

 the use of the cellular arrangement, or diploe, between the 

 two tables of the flat bones, is to give strength, by deaden- 

 ing the force of blows, and, as in the cranium, giving 

 greater security to the organs they are designed to protect. 

 The experiment consisted in taking a certain number of 

 ivory balls and suspending them. When the first in the 

 series was elevated several degrees and let fall against the 

 second, the result was the elevation of the last ball to an 

 angle nearly equal to the first. A ball made of the cellu- 



