ROOTS OF THE CEREBRAL NERVES. 391 



inner and smaller one is motor, the larger outer one 

 sensory in function. 



6. The abducent nerve is very slender, and arises from the 



ventral surface of the medulla, close to the middle 

 line, and immediately behind the pons Varolii. 



7. The facial nerve arises from the outer side of the ante- 



rior end of the corpus trapezoideum, immediately 

 behind the trigeminal nerve. 



8. The auditory nerve is large, and arises from the side 



of the medulla, immediately behind and to the outer 

 side of the facial nerve. 



9 and 10. The glossopharyngeal and pneumogastric nerves 



arise close together, by many slender rootlets, from 

 the side of the medulla, close behind, and a little 

 above the root of the auditory nerve. 



11. The spinal accessory nerve arises from the side of the 



medulla and spinal cord by about ten rootlets, the 

 most anterior of which is just behind the pneumo- 

 gastric, and the hindmost as far back as the fifth 

 spinal nerve. 



12. The hypoglossal nerve arises by a number of rootlets 



from the ventral surface of the medulla, close to the 

 middle line, and to the outer side of the ventral 

 pyramid. 



D. The Structure of the Brain. 



The brain is, from the mode of its formation, tubular, its 

 cavity being directly continuous with the central canal of the 

 spinal cord. From a very early stage it presents a series of 

 dilatations or vesicles, partially separated from each other 

 by constrictions. Of these vesicles, the first or most anterior 

 one is the fore-brain, from which the cerebral hemispheres 

 and thalamencephalon arise as hollow outgrowths ; the second 

 vesicle is the- mid-brain or mesencephalon ; and the succeeding 

 ones, of which there are at least four or five, are spoken of 



