THE NERVOUS SYSTEM 225 



chiefly white matter. It has, however, no convolutions, but is 

 subdivided by many parallel ridges, which, sending down gray 

 matter deeply into the white, central portion, give the latter 

 a somewhat branched appearance.. This peculiar appearance 

 has been called the arbor vitce., or the " tree of life," from the 

 fact that when a section of the organ is made, it bears some 

 resemblance to the trunk and branches of a tree (Fig. 56, F). 

 In size, this cerebellum, or "little brain," is less than one- 

 eighth of the cerebrum. 



13. From the under surface of the cerebrum, and from the 

 front margin of the cerebellum, fibres collect together to form 

 the medulla oblongata (Fig. 57, Ma), which, on issuing from 

 the skull, enters the spinal column, and then becomes known 

 as the spinal chord. From the base of the brain, and from the 

 sides of the medulla originate, also, the cranial nerves, of which 

 there are twelve pairs. These nerves are round cords of 

 glistening white appearance, and, like the arteries, generally 

 lie remote from the surface of the body and are well protected 

 from injury. 



14. THE CRANIAL NERVES 



First Pair. The Olfactory Nerves 



They pass from the base of the brain to the horizontal plate 

 of the ethmoid bone, and send numerous filaments through its 

 many perforations into the mucous membranes of the nose. 

 They convey the sense of smell. 



Second Pair. The Optic Nerves 



They convey the sense of sight, and control the contractile 

 movements of the iris. 



Tliird Pair. The Oculomotorius ** 



It supplies most of the muscles moving the eyeball. 



18. Of what is the medulla oblongata formed ? 

 Of what does it form a part ? 

 Source of the cranial nerves ? How many ? 

 Appearance of the cranial nerves ? Location ? 



