42 CONSIDERATIONS OF THE BRAIN OR ENCEPHALON. 



The Roots of the Twelve Cerebral Nerves (Fig. 21). TJie 

 cerebral nerves (nervi cerebrales) are numbered from before back- 

 ward according to the order of their points of attachment to the 

 brain surface. Those points of attachment are, for the motor 

 roots, points of exit from the brain (apparent origins); and are 

 points of entrance into the brain (apparent central terminations), 

 for all the sensory roots. The genetic nucleus (nucleus originis}, 

 which is the real origin of each motor root, and the terminal 

 nucleus (nucleus terminalis], which contains the real central ter- 

 mination of every sensory root, are imbedded within the brain 

 substance and do not at present concern us. 



1. The olfactory nerves (nervi olfactorii) are the first. .They 

 are the nerves of smell. They are composed of twenty or thirty 

 scattered bundles of non-medullated fibers which rise from the 

 olfactory cells in the nasal mucous membrane and, passing through 

 the cribriform plate of the ethmoid bone, enter the under surface 

 of the olfactory bulb. The surface of the bulb is, therefore, 

 their apparent central termination. The fibers proceed some 

 distance into the gray substance of the olfactory bulb, which 

 constitutes the terminal nucleus of the first nerves, and there 

 branch richly and end in relation with the mitral and bush-cells 

 (real central termination). 



2. Optic Nerve. (Nervus options). The second nerve, the 

 nerve of sight, is really a brain tract rather than a nerve, and 

 its fibers are imbedded hi neuroglia. It rises in the ganglionar 

 layer of the retina. Passing through the chorioid and sclera of 

 the eyeball and the optic foramen of the sphenoid bone, it enters 

 into the optic chiasma where the "nerve" is said to end; but the 

 fibers of the nerve continue without interruption through the 

 optic tracts and their lateral roots to the inter-brain and the mid- 

 brain, whose surfaces they pierce (apparent central termination); 

 they end (real central termination) in the lateral geniculate body, 

 in the pulvinar of the thalamus, and in the superior colliculus of 

 the quadrigeminal bodies. 



3. The oculomotor nerve (n. oculomotorius] is the great motor 

 nerve to the eye (Fig. 21). It issues from the mid-brain at the 

 medial border of the basis pedunculi, which is its apparent origin, 



