NERVOUS SYSTEM. VERTEBRA.TA. 135 



D. 176. The brain of a Great Horned Owl (Bubo maximus). 

 This remarkable brain is distinguished by an immense 

 lateral development of the hemispheres accompanied by a 

 marked approximation of their anterior extremities to the 

 optic chiasma. The enlargement chiefly affects the median 

 part of each hemisphere between the dorsal longitudinal 

 furrow and the mid-dorsal line. By the great development 

 of this area the parts of the hemisphere external to, and 

 including the anterior half of the dorsal longitudinal furrow 

 have been thrust outwards and downwards, so that what 

 under typical conditions would be the lateral parts o the 

 hemisphere occupy a ventral position, and what in other 

 birds would be its dorsal vault forms here its anterior and 

 lateral border. This transformation of parts, accompanied 

 by a marked shortening of the distance between the olfac- 

 tory bulbs and the chiasma, gives to the long axis of the 

 fore part of each hemisphere a nearly transverse direction. 

 The ventral surface of this brain compares well with that 

 of the Goose (D. 167), in which the dorsal parts of the 

 hemispheres have been artificially thrust to either side. 

 In other features, except for the strong development of 

 the optic lobes, the brain is quite typical. 0. C. 1321 K. 



D. 177. A similar specimen, in which the upper part of the left 

 hemisphere has been removed to show that the great lateral 

 expansion of the cerebrum is due to an immense enlarge- 

 ment of the corpus striatum. 0. C. 1321 K a. 



D. 178. The hinder parts of the brain of an Owl (Strix sp.) 

 showing the strong development of the optic lobes and 

 thalami in this keen-sighted bird, and the relatively com- 

 plex formation of the chiasma. In place of the half dozen 

 large interlacing bundles seen in the chiasma of the Duck 

 (D. 169), each optic nerve here splits into five delicate 

 laminae that interlace and alternate s with one another as 

 they pass through the chiasma to the optic tract ot tde 

 opposite side. A certain number of fibres are said also 10 

 pass direct from each optic nerve to the tract of the same 

 side a condition apparently correlated with the frontal 

 position of the eyes. 0. C. 1380 16. 



