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 vertebrae in man are good 

 examples — we have (i) 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, 

 e.g. that by which the thumb articulates with the 

 wrist, or those between the neck vertebrae 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, e.g. 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 



