602 THE BRAIN. 



cinereum, etc. This layer is not continued forward into the lateral ventri- 

 cles of the cerebral hemispheres, but it is well developed backward along the 

 aqueduct (Figs. 136, 137), and in the floor of the fourth ventricle, and through 

 the bulb becomes, as we have seen ( 523), continuous with the central gray 

 matter of the cord. The nerve-cells of this gray matter are on the whole 

 small and in many places scant. 



528. The several roots of the cranial nerves from the third nerve 

 backward may be traced within the brain substance to special collections of 

 gray matter, called the nuclei of the cranial nerves, some of which lie close 

 upon the central gray matter, while others are placed at some distance from 

 it. The optic nerve and what is sometimes called the olfactory nerve, namely, 

 the olfactory bulb and tract, may advantageously be dealt with apart, since 

 these two nerves are not, like the other cranial nerves, simple outgrowths 

 from the walls of the original neural canal, but are in reality elongated 

 vesicles, budded off from the neural canal, the cavities of which have been 

 obliterated. We may add that part of the retina, and of the gray matter 

 of the olfactory tract, may perhaps be considered as corresponding to the 

 nuclei of which we are speaking, the retinal and proper olfactory fibres being 

 connected with them very much as the fibres of the remaining cranial nerves 

 are connected with their respective nuclei. In the brain the segmental reg- 

 ularity 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 may be seen in a ventral view of the brain (Fig. 131, (7.), 

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

 origin ;" the third nerve issuing in front of the pons, and the last or hypo- 

 glossal stretching back to the hind end of the bulb. Part, indeed, of the 

 eleventh nerve, the spinal accessory nerve properly so-called, makes con- 

 nections 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 connec- 

 tions be distinguished from the remaining part of the nerve, as well as from 

 all 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 further. Fig. 138 is a diagram showing in a roughly approxi- 

 mate 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. 



529. The twelfth or hypoglossal nerve. The nucleus of this nerve, which 

 it will be convenient to take first (Fig. 138, XII.), is a long column of gray 

 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. 132, 2) 

 it occupies a ventral position and is a part of the anterior horn ; thence 

 it gradually rises dorsally (Fig. 132, 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 



