THE LIVING BIRD 27 



limbs, which have been entirely devoted to flight. 

 (In the Hoatzin the forelimbs are still used for 

 climbing, in ancestral fashion, during nestling life). 



This combination of requirements occurs nowhere 

 else in the animal kingdom and in consequence the 

 bird has a skull that is peculiar and unique. 



In its essentials the nasal organ of birds resembles 

 that of other vertebrates. The acute sense of smell 

 in mammals is a familiar fact. The relatively great 

 distance between the large, sensitive nostrils, placed 

 at the end of a ''snout," and the olfactory lobes of 

 the brain, may not be so familiar but it constitutes, 

 nevertheless, an important corollary for it is in this 

 intervening space that the large tract of sensory epi- 

 thelium, responsible for picking up the olfactory 

 stimuli, is situated. Reduction of the cavity inevi- 

 tably means reduction in smelling ability. The 

 olfactory mucous membrane in mammals (exclusive 

 of the whales and their immediate relatives) is very 

 extensive, the available surface in the anterior end 

 of the skull being enormously increased by the 

 development of thin sheets of scrolled bone, (turbi- 

 nals), covered with the nasal epithelium. Thou- 

 sands of special sense cells here receive olfactory 

 stimuli at each intake of breath, the sensations being 

 transmitted to the olfactory regions of the brain via 

 exclusive nerve fibres. In the case of birds, in 



