342 The Outline of Science 



cells, gland-cells, and so on ; and they differ remarkably in struc- 

 ture from each other. 



One contingent of these cells consists of the "bone-builders," 

 and long before birth they begin to construct the supporting 

 framework of the body. It is, of course, not bone at first. 

 Frames of cartilage preceded bony frames in the course of racial 

 evolution, and a cartilage-frame goes before bone in the develop- 

 ment of the individual body. When the time comes, the bone- 

 builders extract the lime-salts which have got into the blood with 

 the digested food, and they use these in building up the bones. 

 Sir Arthur Keith tells us that there are two million of these 

 bone-builder cells at work in the thigh of a new-born baby, 

 and that the number rises later to a hundred and fifty millions. 

 They make the bones solid, then they change the interior 

 into the light but strong texture with which everybody is 

 familiar. 



How is it that we feel no creaking, no jarring, or friction, 

 of the two hundred and thirty joints by which our bones play 

 upon one another? Here is another ingenious contrivance. A 

 layer of cartilage remains over the end of each bone. It is dense, 

 very elastic, and always well lubricated by one of the many re- 

 markable automatic lubricating systems of the body. The cart- 

 ilage cells themselves in this case are converted into lubricating 

 fluid when they die! 



The muscular system which moves the bones is the red flesh 

 with which we are familiar in the butcher's shop. Everybody 

 who has carved a joint, and knows the importance of cutting 

 "against the grain," is aware that one of these large Auscles of 

 the ribs or limbs of a cow consists of muscular fibres packed 

 closely in bundles. There are 600,000 fibres in a single muscle 

 of man's arm, the biceps. Each fibre is composed of many fibrils, 

 the seat of that power of contractility which we very little under- 

 stand. The body-machine is still full of problems and mysteries 

 for us. Three hundred years ago the courageous anatomists of 



