CRANIAL NERVES 



795 



The oculomotor or third pair of cranial nerves make their exit from the poste- 

 rior perforated substance in the interpeduncular fossa just behind the corpora 

 mammillaria. 



\J The trochlear nerves emerge around the lateral aspects of the pedunculi 

 cerebri along the anterior border of the pons. The trochlear is the smallest of the 

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



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. It is a purely sensory 

 nerve, but it is accompanied by the much smaller masticator nerve which is motor 

 and is usually referred to as the motor root of the trigeminus. 



Fig. 625.^View of the Base of the Brain. (After Beaunis.) 



Tri- 

 geminus 



Abducens 



Facial 

 Acoustic 

 Glosso- 

 pharyngeal 

 Vagus 

 Spinal 

 accessory 

 Hypo 

 glossal 



Gyri orbitales 



Anterior perfor- 

 ated substance 



Hypophysis 



tuber 



Cinereum 



Mammillary 

 body 



Post. perf. 

 substance 



Cerebral 

 peduncle 



Pons (Varoli) 



Pyramid 

 Olivary body 



Tonsil 



(amygdala) 



Section of 

 spinal cord 



Inferior vermis 



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

 border of the pons: — the abducens nerve, which is motor, emerges near the 

 mid-line; the facial, motor, emerges from the more lateral aspect of the brain- 

 stem; the glosso-palatine or the intermediate nerve of Wrisberg, largely sensory, 

 is attached in company with the facial; and, entering the extreme lateral aspect 

 of the stem are the cochlear and vestibular nerves. These latter two, when taken 

 together as one, are known as the acoustic (auditory) or eighth cranial nerve. 

 They are both purely sensory. The cochlear courses for the most part laterally 

 and dorsally around the inferior cerebellar peduncle, giving it the appearance 

 from which it derives its name, 'restiform body.' 



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

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

 the inferior border of the pons above, and continuous with the first segment of 

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

 bodies) are the two most prominent structures. The pyramids, which are con- 

 tinuous below into the pyramidal (cerebro-spinal) tracts of the spinal cord, form 

 the two tapering prominences along either side of the anterior median fissure; the 

 olives are the oblong oval elevations situated between the pyramids and the resti- 



