478 



THE BIRDS 



xvi. 27 



XII 

 IX,X,XI 

 Vllanc/VIII 



The cerebellum is also large (Fig. 288), a state of affairs perhaps 

 connected with the precise timing and control of movement in all 

 planes of space during flight. Besides large spino-cerebellar and 

 vestibulo-cerebellar pathways there are also tecto-cerebellar and 

 strio-cerebellar tracts, the latter perhaps conducting in both directions. 

 The effect of the cerebellum on other parts of the brain is exercised 

 through cerebellar nuclei, the cells of which give origin to the cerebello- 

 spinal tract. 



The optic tracts are completely 

 crossed and end mainly in the 

 midbrain, as in lower verte- 

 brates. However, a considerable 

 portion of the optic tracts passes 

 to the thalamus, and the mid- 

 brain and thalamus are both 

 highly developed and have inti- 

 mate and reciprocal connexions 

 with the striata of the cerebral 

 hemispheres. The optic lobes 

 also receive ascending fibres from 

 the trigeminal nuclei and from 

 the spinal cord. Their efferent 

 pathways run to the oculomotor 

 nuclei, to the underlying teg- 

 mentum, and to the medulla and 

 spinal cord. Evidently they play 

 a large part in correlating visual with other afferent impulses. The 

 thalamus is large and its dorsal part well differentiated into nuclei. 

 It receives, besides optic fibres, also projections from tactile, pain, 

 temperature, and perhaps auditory sources. There are large thalamo- 

 striatal tracts, probably conducting in both directions. The ventral 

 thalamus receives impulses from the striatum and sends them to the 

 tegmentum, this being the main efferent pathway of the forebrain. 

 The hypothalamus is rather small, probably because of the reduction 

 in the olfactory system. 



The cerebral hemispheres are much larger than any other part of the 

 brain and show an exaggeration of the condition found in the lizards 

 (Fig. 289). The ventro-lateral portions are enormously developed, 

 whereas the medial ventral walls are thin and the pallium is quite 

 small, thin, and not folded. The olfactory regions of the brain are 

 small, including the hippocampus. 



Fig. 288. Brain of a duck (Anser) 



c.h. cerebral hemisphere; cereb. cerebellum; cp. 



epiphysis;/?, flocculus; h. hypophysis; o./. optic 



lobe; olf. olfactory lobe; str. striatum; 1I-XII, 



cranial nerves. (After Butschli and Ihle.) 



