78 
MAMMALIAN ANATOMY 
trapezoid bodies the pyramids are flanked laterally by the 
rounded elevations known as the olivary bodies. 
Nerve Roots. —(These are variously made up of sensory and 
motor components, indicated below respectively by the letters S 
and M. In certain cases, however, in which a nerve is designated 
as motor, small sensory components may also be present.) 
Olfactory (S) from 1 the olfactory lobes. These consist of 
many small bundles which are always torn from the olfactory 
lobes in removing the brain. 
Optic (S) from the optic chiasma. 
Oculomotor (M) appearing from the ventral surface of the 
cerebral peduncles. 
Trochlear (M) appearing laterally between the mesen¬ 
cephalon and the metencephalon, but arising from the dorsal 
surface at the posterior border of the mesencephalon. 
Trigeminal (M) appearing from the lateral border of the pons. 
Abducent (M) appearing from the lateral boundaries of the 
pyramids slightly posterior to the pons. 
Facial (S and M) from the latero-posterior borders of the 
trapezoid bodies. 
Acoustic (S) appearing laterally posterior to the pons. 
Glossopharyngeal (S and M) lateral to the olivary bodies. 
Vagus (S and M) from the lateral surface of the medulla 
posterior to the glossopharyngeal. 
Accessory (M) from many roots along the lateral surface of 
the spinal cord and medulla, these roots uniting and extending 
anteriorly as far as the root of the vagus, which they then accom¬ 
pany out of the cranial cavity. 
Hypoglossal (M) from the posterior region of the medulla 
along the lateral margins of the pyramids. 
B. INTERNAL STRUCTURE. 
i. Median Sagittal Section. 
Place the brain with the ventral surface up upon a layer of 
cotton on the table, and, using as a guide the midventral fissure, 
1 For convenience of description we may speak of both sorts of nerve roots as 
coming from the brain but in reality only the motor components actually originate 
from the brain, while the sensory ones grow into the brain from ganglia or from the 
sense organs themselves. 
