‘194 THE STRUCTURE AND LIFE OF BIRDS CHAP. 
Passive Machinery. 
The Passive Machinery consists of bones, ligaments, 
tendons, membranes and feathers. None of these 
have, like muscular tissue, any power of contraction. 
.Any activity they-may show is really due to their 
being acted on directly or indirectly by muscles. 
‘’ It will be well first to make some general remarks on 
‘the different kinds of joints. Passing over what are 
called imperfect joints because they allow-of very little 
motion—those between the vertebra in man are good 
examples—we have (1) ball and socket joints where the 
rounded surface of one bone fits into a cuplike hollow 
in another ; (2) hinge joints where only motion back- 
wards and forwards is possible ; (3) double hinge joints, 
eg. that by which the thumb articulates with the 
wrist, or those between the neck vertebra of birds; I 
compared this joint above to two saddles put one 
atop of the other; it has been well likened to the 
articulation between the rider and the saddle ; (4) pivot 
joints, eg. that between the skull and the atlas 
vertebra. 
In the wing we have examples in full working order - 
only of the ball and socket, and hinge joints. The 
shoulder joint comes under the former head, and yet 
it is very different from the types of which we have a 
familiar instance in the thigh joint, in which a round 
ball fits into a deep cup. At the shoulder the cup is 
shallow and imperfect, and the bone which fits into it 
is not round. The result is that it works with great 
freedom, so much so that it has sometimes been called 
