vi FORM AND FUNCTION 143 



Muscle is related to another kind of tissue which yet 

 in its function is very different. Tendons have no 

 power of contraction. They are . merely cords by 

 which, in many cases, the ends of muscles are 

 fastened to the bones. In youth there is compara- 

 tively little tendon in the body, nearly all is muscle, 

 and to this is due the springiness of the limbs. In 

 age one of two things happens : either the muscle 

 undergoes a kind of degeneration, fat making its way 

 in among the tissue, as we often see it, in small streaks 

 and flecks, in beef; or else the tendon by which the 

 muscle is attached grows longer, while the muscle 

 grows shorter, an increasing stiffness being the inevit- 

 able result. Long tendons, for quite different reasons, 

 to be explained soon (see p. 208), are characteristic of 

 birds. When, as they move, they have to rub against 

 hard surfaces of bone, tendons are protected by little 

 bags filled with moisture ; sometimes they are com- 

 pletely sheathed at these points. Sometimes their 

 working makes grooves in the bones. This can be 

 well seen at the ankle-joint of birds or where the toes 

 spring from the metatarsals. Tendons themselves, 

 in some cases, change their nature, and become 

 sesamoid bones as they are called. Such bones are, 

 for instance, the knee-cap, the pisiform bone, a small 

 bone that can be felt on the outer side of the wrist, 

 and the marsupial bones of the kangaroo. 



Bones. 



Much has been said on this subject in the opening 

 chapters (see especially Chapter II.), and in the 



