THE SKELETON 263 



lengthwise of the bone, and containing blood vessels have 

 been cut off; the concentric layers of bone tissue about 

 these are called bone lamellae. In the living bone these 

 central openings are occupied by blood vessels. Arranged 

 in rings around the vessels between the scale-like layers of 

 the lamella?, are a large number of small, lens-shaped spaces, 

 called lacunae, each of which shows numerous fine lines radia- 

 ting from it. Row after row of these little spaces appear 

 arranged around the central blood vessel, forming larger and 

 larger rings until they reach similar rings belonging to other 

 centers. 



Each of the little spaces in the live bone is filled with living 

 matter, which is the remainder of what was originally a bone 

 cell. Because of the deposit of such mineral matter about it, 

 the original shape of the cell has become almost unrecogniz- 

 able. The radiating lines are really minute tubes, canaliculi, 

 passing from one row of spaces to the next. Since each row 

 is connected with the next, and since the inner row, by means 

 of the little tubes, is connected with the central space and 

 hence with the blood vessel, even the outer row of spaces is 

 supplied with nourishment derived from this vessel. It is be- 

 cause the bone possesses so many living cells, so well supplied 

 with nourishment, that it can be so easily repaired. 



CARTILAGE 



We have noticed that the ribs are not united directly to 

 the sternum, but that there is a short piece of softer material 

 at their front ends; this material is cartilage, a substance 

 which forms an important part of the skeleton. Early in 

 life most of the bones are made of cartilage. Even in the 

 adult some cartilage still remains; e. g. the cushions between 

 the vertebrae, the supporting pieces around the windpipe 

 (Fig. 98), the pieces at the ends of the ribs (Fig. 101), in the 

 joints, and in the outer ear, which consists of cartilage covered 

 with skin. Cartilage, which is flexible but very tough, is 



