n6 THE STRUCTURE AND LIFE OF BIRDS chap. 



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 when a 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 



