64 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, ze 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 twelve 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. NV. olfactorius 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. 
Il. NV. 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. 
Ill. WV. oculomotorius arises close behind the hypophysis, near 
the medio-ventral line, from the midbrain, enters the orbit behind 
or together with 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. NV. 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. 
