84 THE ENGINES OF THE HUMAN BODY 



of the two opposed bones are coated with a thin layer of 

 cartilage, less than an eighth of an inch in thickness. 

 We notice, too, that this covering of cartilage is most 

 firmly fixed to the terminal surfaces of bones ; indeed, 

 as we have already seen, it is part of the end cartilage 

 which has survived the onslaught of the invading osteo- 

 blasts. One observes, also, when one presses on the carti- 

 lage that it yields or flattens, but when the pressure is 

 withdrawn it springs out again, showing that the carti- 

 lage which lines joints is elastic and can serve as a buffer. 

 Its surface, too, is always covered by a film of substance 

 named synovia, which is not unlike white of egg in ap- 

 pearance, but so slippery that cartilage slips through our 

 fingers should we try to grasp a bone by its articular end. 

 As for colour, joint cartilage is pearly-white, there being 

 no tinge of red in it, for it is perfectly bloodless. Not a 

 vessel, not even a red blood corpuscle, crosses its threshold. 

 And yet the cartilage which lines every joint of the body, 

 as well as the ankle-joint, is alive. We have seen that 

 cartilage is studded with living microscopic units which 

 surround themselves with thick, soft walls of that rubber- 

 like substance which anatomists have named chondrin. 

 For that reason cartilage corpuscles must be provided 

 with sustenance, for all things which are actively alive 

 must be constantly nourished. 



In order to see how cartilage and " cartilage-builders " 

 live and work as engineers and lubricators, a small part 

 of the articular cartilage of the ankle-joint indicated in 

 fig. 23 has been magnified fifty times and drawn in fig. 24. 

 The spaces in the bone, immediately under the attached 

 surface of the thin cartilage plate, are richly supplied with 

 blood, yielding more than can be needed by the neigh- 

 bouring bone-builders. Hence we suppose that the 

 cartilage-builders draw their nourishment from the vessels 

 of the underlying bone, imbibing merely the fluid part of 

 the blood — its plasma. We notice, too, that the cartilage- 

 builders have arranged themselves in three tiers or sets 

 within the articular plate (fig. 24). In the middle stratum 

 of the cartilage plate they are loosely grouped in irregular 



