XXIV] BRAIN 619 



as befits an animal which flies. Similar air spaces also exist in the 

 larger bones of the trunk and limbs. The insects, which also have 

 taken to the air, have somewhat analogous air reservoirs. Like insects, 

 birds are represented by a large number of species which all exhibit 

 great uniformity of structure. 



When the brain is examined, the meaning of many of the 

 The Brain peculiarities of the skull is seen. What we might 

 perhaps, with a little looseness, call the organs of 

 thought, the hemispheres of the fore-brain, are greatly enlarged, 

 being high and rounded, but sections reveal the fact that the 

 great mass of the hemisphere is composed of an enlargement of the 

 base which corresponds to what is called corpus striatum in the 

 mammalian brain. The roof of the hemisphere corresponding to 

 the cortex in Mammalia is thin. Now in Mammals the corpus 

 striatum is generally regarded as the seat of those impulses which 

 carry out the instinctive activities, whereas the cortex is the seat 

 of purposive action. In accordance with their brain-structure, we 

 find that Birds are creatures of instinctive impulse, and have not 

 nearly so much intelligence as they are usually credited with by 

 imaginative people. One instance may suffice. The cuckoo nest- 

 ling forces its foster parents to feed it by uttering a peculiarly 

 plaintive cry. The impulse to provide the food is a mechanical 

 reaction to this cry, for if the foster parents be shot, the cuckoo 

 nestling will arrest the flight of other parent birds on their way to 

 their nests and cause them to neglect their own young and feed the 

 voracious cuckoo till they sometimes drop dead of exhaustion. 

 The parts of the brain supplying the nose, the olfactory lobes, 

 are very small and poorly developed, in accordance with the feebly- 

 developed nasal sacs, the sense of smell being but slight, as men- 

 tioned above (Fig. 308). The brain is bent sharply on itself, so 

 that the optic lobes of the mid-brain portions connected largely 

 with vision are pressed downwards and the hemispheres are 

 brought close to the cerebellum, which, in contradistinction to 

 what is the case in most reptiles, is large and transversely wrinkled. 

 Evidence is accumulating that an important function of the cere- 

 bellum is to coordinate the motor impulses to the skeletal muscles 

 which bring about the correct balance of the animal. As balance is 

 a more difficult matter in a bipedal animal than in a quadruped the 

 cerebellum of birds is correspondingly enlarged. 



All Birds have comparatively long necks -(Fig. 307), and the 

 vertebrae which form the support of this part of the body have the 

 surfaces with which they articulate with one another shaped like 



