954 NUCLEI OF CRANIAL NERVES. [BOOK m. 



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 

 manner. They maybe 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 pyra- 

 mids, 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, xn.), 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 



