778 



THE NERVOUS SYSTEM 



smallest of the cranial nerves, and the only pair arising from the dorsal aspect of 

 the brain. They undergo total decussation in the region of their exit, just behind 

 the posterior quadrigeminate bodies. 



The trigeminus, or fifth cranial nerve, is the largest. It penetrates the pons to 

 find its recipient nuclei in the depths of the brain-stem. For the most part it is a 

 sensory nerve, but it is accompanied by a small motor root (portio minor). 



Three pairs of cranial nerves are attached to the brain-stem along the inferior 

 border of the pons: the abducens or sixth nerve emerges near the mid-line; the 

 facial or seventh emerges from the more lateral aspect of the brain-stem; and the 

 acoustic or eighth enters the extreme lateral aspect of the stem. The eighth con- 

 sists of two parts, a vestibular and a cochlear division. The cochlear division courses 

 for the most part externally and dorsally around the inferior cerebellnr peduncle, 

 giving it the appearance from which it derives its name, ' restiform body.' 



FIG. 576. VIEW OF THE BASE OF THE BRAIN. (After Beaunis.) 



OLFACTORY 

 BULB 



OPERCULVU 



3d or OPTIC 

 XERVB 



id or OCULO- 



MO TOR 



Uh or TROCH- 



LEAK 



Btk or ABDUCEffS 



rtk or FA CIAL 



SOt or ACOUSTIC 



<tth. GLOSSO- 



PHARYXGEAL 



mk, rA 



nth, SPIRAL 



ACCESSORY 



lit*. HYPO- 



GLOSSAL 



GTRI 



ORB1TALES 



HYJ'OPHYSIS 

 Ti'BKR 



ClSKREl'M 

 MAM.VILI.ARY 



lilt in' 



POST. PERP. 



SrBSTA.VCS 

 CKIIK11RAL 



The remaining four pairs of the cranial nerves are attached directly to the 



3dulla oblongata. This comprises that portion of the brain-stem beginning at 



ie posterior border of the pons above, and continuous with the first segment of 



spinal cord below. On its ventral surface the pyramids and the olives are 



the two most prominent structures. The pyramids, which are continuous below 



into the pyramidal tracts of the spinal cord, form the two tapering prominences 



along either side of the anterior median fissure; the olives are the oblong oval 



levations situated between the pyramids and the restiform bodies, and each is 



the superficial indication of the inferior olivary nucleus. 



The glosso-pharyngeal or ninth nerve, the vagus (pneumogastrio) or tenth, and 



the spinal accessory or eleventh cranial nerve are attached along the lateral aspect 



.e medulla oblongata in line with the facial nerve and between the olive and the 



corpus restiforme. The spinal accessory is assembled from a series of rootlets which 



