116 THE STRUCTURE AND LIFE OF BIRDS cwap. 
tinguishable by its whiteness. Upon the lungs alone 
must fall the duty of getting rid of superfluous water 
in any large quantity. 
The Nerves. 
About the nerves it is unnecessary to say much, 
since they do not differ very materially from those of 
mammals. The spinal cord is the great trunk nerve 
which sends out branches on either side between the 
vertebra. It broadens out and forms part of the back 
of the brain. There is also another system.of nerves 
called the Sympathetic, which lies in front of the verte- 
bral column, and which acts mainly on the intestines 
and blood vessels, not on the voluntary muscles. It is 
connected with the spinal cord and so with the brain. 
Nerves are called afferent and efferent. When any part 
of the body comes into contact with anything that 
necessitates prompt action—for instance, red-hot iron— 
the afferent nerve carries the news to the spinal cord, 
and so to the brain. The efferent nerve causes the re- 
quisite muscular movements. In every warm-blooded 
animal the nerves are highly developed. Otherwise 
a highly-organised brain would be of little use. The 
keenness of sight and hearing for which birds are 
remarkable shows the perfection of their nervous 
system. Great strength may co-exist with sluggish 
nerves, as in a crocodile. But whena Swallow catches 
sight of a gnat, and in less than a second has 
taken all the necessary steps—eye communicating 
with brain, brain directing the proper adjustment 
