54 BRAIN 



the weight of the hemispheres with that of other parts of the cen- 

 tral nervous system, in order to draw conclusions as to the intelli- 

 gence of various Birds. When Birds are arranged according to the 

 preponderance of the hemispheres over the rest of the brain, the first 

 place is taken by the Passeres and Parrots (2*7 or 2*0 to 1), then 

 follow Geese, Ducks, Waders, and Birds -of -Prey, lastly Fowls and 

 Pigeons, the proportions in the Common Domestic Pigeon being 

 0"95 to 1, i.e. the forebrain weighs less than the rest, while in many 

 Oscines it weighs nearly three times as much. The attempts to 

 sort Birds according to the proportion of brain to body have led 

 to no practical results, chiefly because the variable conditions of fat 

 and lean subjects have not been considered. The absolute weight 

 or mass alone of the brain is not a safe guide. 



There are twelm pairs of cranial or brain-nerves which arise from 

 the brain and leave the cranium through special holes. These 

 pairs, as in other Classes of Vertebrates, are frequently spoken 

 of by their number, counting from the nasal region backwards 

 to the occiput. 



I. N. olfadorius forms the anterior and ventral continuation 

 of the hemisphere of its side, but arises in reality from ganglionic 

 cells in the thalamencephalon and the midbrain. It leaves the 

 cranial cavity through a canal in the dorsal and median part of the 

 orbit and ends in the ganglionic cells of the olfactory membrane of 

 the nose. 



II. iV". opticus arises from the ganglionic cells of the mantle 

 of the optic lobes. Immediately in front of the hypophysis is the 

 optic chiasma, produced by the complete crossing of the fibres 

 which compose the two optic nerves, those from the right optic 

 lobe passing over the left, and those from the left lobe to the right 

 side. From the chiasma start the right and left optic nerves, each 

 leaving the cranium by the large optic foramen between the orbito- 

 sphenoid and alisphenoid, entering the orbit near the posterior and 

 ventral corner of the orbital septum and ultimately forming the 

 retina of the eye. 



III. N. oculortiotmius arises close behind the hypophysis, near 

 the medio-ventral line, from the midbrain, enters the orbit behind 

 or together Avith the optic nerve (II), and supplies most of the ex- 

 ternal muscles of the eye, namely the m. rectus superior, inferior, 

 internus, and obliquus inferior. A ciliary, partly sympathetic, 

 branch supplies the eyeball and the internal muscles (see Eye). 



IV. N. trochlearis or patheticus is the only one which leaves the 

 brain on its dorsal surface, namely as a thin thread winding its way 

 from the midbrain upwards between the cerebellum and the optic 

 lobes, and entering the orbit through a fine opening close to the 

 optic nerve (II) in order to supply the m. obliquus superior of the 

 eyeball. 



