20 THE STRUCTURE AND LIFE OF BIRDS cuap. 
utmost importance. Otherwise, how could the head 
be supported at the end of so long a neck? But 
muscles get tired with prolonged exertion, however 
slight the exertion may be, and to provide against 
this there are, between the spines of the vertebre of 
the neck (SP, fig. 2), elastic ligaments similar to that 
which is so enormously developed in the horse’s neck 
to support his ponderous head. These ligaments. hold 
the neck in position when it forms an §. Inthe Swan 
they are but slightly developed, hence perhaps the 
ease with which he erects his neck straight as a flag- 
staff. Even when there are ligaments to relieve the 
muscles, the skull is the place where aération of bones 
is desirable if anywhere. Its great size compared with 
that of the lizard, and, consequently, the great size of 
the brain, I have already pointed out. The skull, too, 
illustrates better than any other part of the skeleton 
the tendency to ankylosis, or fusion of bones. Even 
in a very young bird this has already proceeded a long 
way. The skull seems to be made up of a shell of 
bone almost without suture. It is really composed of 
scores of different bones, the boundary lines between 
which may be seen in the embryo. And how are these 
to be studied? It is possible to go through them and 
learn them up as one does for an examination. But 
for such studies it is usually the imminence of the 
examination, not the interest of the subject, which 
supplies the stimulus. Without this stimulus, to a 
mind that has not as yet the patience wanted for 
scientific investigation—the patience to collect facts, 
even if the clue to them and the interest of them may 
not be found till years after—there is something barren 
