954 NUCLEI OF CRANIAL NERVES. [BOOK in. 



In the brain, the segmental regularity of the nerve roots so 

 conspicuous in the spinal cord is very greatly obscured. We shall 

 have something to say on this point later on ; but at present we 

 may be content to treat the several nerves in a simple topographical 

 mariner. They may be seen in a ventral view of the brain Fig. 108, c 

 leaving the brain at various levels by what is called their " super- 

 ficial origin ; " the third nerve issuing in front of the pons, and 

 the last or hypoglossal stretching back to the hind end of the bulb. 

 Part, indeed, of the eleventh nerve, the spinal accessory nerve 

 properly so called, makes connections with the spinal cord below 

 the bulb as far back as the sixth or seventh cervical nerve, or even 

 lower; but this part may by these connections be distinguished 

 from the remaining part of the nerve, as well as from all the other 

 cranial nerves. The nuclei to which the nerve roots may be traced 

 within the brain substance, sometimes spoken of as the "deep 

 origin," range in position from the hinder part of the bulb to the 

 hind end of the third ventricle. The nucleus of the hypoglossal 

 nerve begins in the bulb just above the decussation of the 

 pyramids, the nucleus of the third nerve ends beneath the hind 

 end of the floor of the third ventricle ; and all the rest of the 

 nuclei may be broadly described as placed between these limits 

 in various parts of the floor of the central canal or in adjoining 

 structures, though part of one nucleus, namely, that of the fifth 

 nerve, can be traced, as we shall see, back into the spinal cord as 

 far as the second cervical nerve and probably extends still farther. 

 Fig. 115 is a diagram shewing in a roughly approximate manner 

 the nuclei of the several nerves as they would appear in a bird's- 

 eye view of the floor of the aqueduct and fourth ventricle looked 

 at on the dorsal aspect. 



616. The Twelfth or Hypoglossal Nerve. The nucleus of this 

 nerve, which it will be convenient to take first (Fig. 115, XII.), is 

 a long column of grey matter lying in the bulb parallel to, and 

 very close to, the median line. It reaches from the hinder part 

 of the fourth ventricle, at about the level of the hind end of 

 the auditory nucleus, as far back as beyond the hind end of the 

 olivary body. At its extreme hind end or beginning (Fig. 109, 2), 

 it occupies a ventral position and is a part of the anterior 

 horn ; thence it gradually rises dorsally (Fig. 109, 3, 4, 5), but so 

 long as the central canal remains closed continues to occupy a 

 distinctly ventral position in reference to the central canal ; in 

 its front part, it is, by the opening up of the fourth ventricle, 

 brought into an apparently more dorsal position (Fig. 109, 6). 



The nucleus consists mainly of large nerve cells with distinct 

 axis-cylinder processes, which though pursuing a somewhat irre- 

 gular course may be traced into the fibres of the nerve. These, 

 starting from the ventral surface of the nucleus along its length, run 

 ventrally through the reticular formation, and making their way in 

 a series of bundles, between the olivary nucleus on the lateral side 



