XVI. 28 



FUNCTION OF BRAIN 



479 



The corpus striatum is a huge solid mass of tissue, receiving projec- 

 tions forward from the thalamus and sending them back through the 

 latter, to the midbrain roof and floor, to the cerebellum, and thence to 

 the medulla and spinal cord. This very characteristic striatum can be 

 divided into various regions. The part representing the 'original' or 

 lower striatum is called the 'paleostriatum' ; other parts, lying above 

 this, are known as the mesostriatum and hyperstriatum. 



Fig. 289. Transverse section through forebrain of sparrow. 



hip. hippocampus; hyp.str. hyperstriatum; mes.str. mesostriatum; n.pr. preoptic nucleus; 

 pal.str. palaeostriatum; pall, pallium. (Partly after Kappers, Huber, and Crosby.) 



28. Functioning of the brain in birds 



Loss of one complete hemisphere by a pigeon is not followed by any 

 gross motor defect or asymmetry of movement. This suggests that the 

 corpora striata do not control individual muscle movements, which 

 agrees with the fact that there is no direct pathway from the forebrain 

 to the spinal cord, corresponding to the pyramidal tract of the mam- 

 mals. Electrical stimulation does not produce movements; the striata 

 are 'silent areas' to stimulation. 



Complete removal of both hemispheres does not reduce a pigeon to 

 a helpless state. The animal can still maintain its temperature and its 

 balance and can feed itself if the food is placed near to it. However, a 

 bird so treated is far from normal. It may show a lack of activity, 

 remaining inert for long periods, and then become aimlessly restless 

 for a while. Evidently the normal balance of excitation and inhibition 

 has been upset. Deficiencies in vision can be detected in birds with 

 various portions of the cerebral hemispheres removed, and the mating 

 and nestine behaviour are also affected. Even small removals of the 



