144 THE STRUCTURE AND LIFE OF BIRDS cuap, 
present one under the head of “Hollow Bones.” 
For what remains the reader is referred to the re- 
marks on “Passive Machinery” in the next chapter. 
Ligaments. 
Ligaments are like tendons in having no power of 
contraction, but, unlike tendons, they are not con- 
nected with muscles. Their usual function is to 
fasten two bones together at the joint, and to limit 
the amount of freedom with which one turns upon 
the other. When a skeleton is obtained by macera- 
tion—ze., by leaving the carcase in water till the flesh 
is easily removable, many of the ligaments still re- 
main and keep the bones in their proper connection. 
There are some which answer very different purposes. 
The horse’s head is supported by a strong elastic liga- 
ment attached to the upright spines of the vertebra. 
The bird, as I shall show in the article on “ Passive 
Machinery” in the next chapter, has several which 
are remarkable for their elasticity, some, if not all, of 
.these having been originally tendons. 
Feathers—Structure and Development. 
A feather is a very elaborate appendage. When 
we are told that a Peacock’s or an Ostrich’s plume, or 
the wing-feather of an Albatross is an “ epidermic 
growth,” part, that is, of the horny outer skin, we 
seem to hear words that explain nothing. There is 
another “epidermic growth,” the nature of which it is 
perhaps hardly less difficult to realise—the horn of a 
