The Central Nervous System. 277 



(e) On the ventral surface (Fig. 85), the pons forms a broad 

 . comrnissural band extending transversely across the brain 



and upward into the supports of the cerebellum. Its surface 

 is divided into two parts by a median depression, the sulcus 

 basilaris, occupied by the basilar artery. 



(f) The anterior median fissure of the spinal cord ends at the 

 pqsterior margin of the pons in a faint depression, the fora- 

 men caecum. 



(g) The anterior funiculus of the cord is largely replaced for- 

 wards by the pyramid (pyramis) , a narrow band extending 

 forward on either side of the middle line to the posterior 

 margin of the pons. 



(h) The trapezoid body (corpus trapezoideum) lies in the 

 angle formed by, the lateral margin of the' pyramid with the 

 posterior border of the pons. 



(i) The fifth cranial, ox trigeminal nerve (n. trigeminus), 

 arises by two roots, a larger sensory root, the portio major, 

 and a smaller motor root, the portio minor. The two 

 parts appear at the lateral border of the pons. 



The portio major is the common trunk of the ophthalmic, maxillary 

 and mandibular nerves, the portio minor joining the mandibular, so 

 that the latter becomes a mixed nerve. The cut end of the portio major 

 may be identified on the cranial wall arbd traced forward into the semi- 

 lunar ganglion, the latter lying in a depression at the anterior ventral * 

 end of the petrosal bone. 



(j) The sixth cranial, or abducent nerve (n. abducens), is a 

 slender cord arising at the antericfr end of the pyramid. 



(k) The seventh cranial, or facial nerve (n. facialis), and the 

 eighth, or acoustic nerve (n. acusticus), arise from the 

 lateral margin of the trapezoid body. 



The two nerves are closely associated, the former being slightly 

 anterior in position. Its chief portion is the motor root. In addition 

 the nerve receives a sensory filament, the portio intermedia or inter- 

 mediate nerve. 



(1) The glossopharyngeus, vagus, and accessorius arise by 

 several roots arranged in a linear series along the lateral 

 margin of the medulla. The trunk of the accessorius ex- 

 tends backward on the spinal cord, its roots, about ten in 

 number, arising as far back as the fifth cervical spinal nerve. 



(m) The twelfth cranial, or hypoglossal nerve (n. hypoglossus) , 

 arises by several roots from the ve'nti-al surface of the medulla 



