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 
