54 LABORATOEY OUTLINE OF NEUROLOGY 



deeper. In mammals some of these nuclei appear on the 

 ventricular floor, but most of them lie too deep to be located 

 by surface study. The ala cinerea (or trigonum vagi) is an 

 eminence which marks the position of the dorsal motor nucleus 

 of the vagus (general visceral efferent) The special visceral 

 nucleus of the IX and X nerves (nucleus ambiguus) is not 

 visible from the surface. Similarly the visceral motor nuclei 

 of the VII and V nerves lie too deep to be marked on the floor 

 of the ventricle, though root-fibers from the motor VII nucleus 

 form a curious knee-shaped bend (the genu) which forms a 

 part of the colliculus facialis. 



54. Sulcus limitans. In embryonic brains there is a longi- 

 tudinal limiting sulcus which separates the ventro-medial 

 motor columns from the dorso-lateral sensory columns. In 

 the brain of the adult sheep this sulcus is preserved for the 

 entire length of the fossa rhomboidea. It is much deeper in 

 two places than elsewhere, thus forming the fovea superior and 

 fovea inferior (Fig. 11). The same relations are sometimes 

 found in the adult human brains, though here the middle part 

 of the sulcus limitans is often obliterated by the vestibular 

 nucleus and striae medullares acustici. 



55. Visceral sensory column. In the dogfish (Section 23) 

 the visceral sensory nuclei of the cranial nerves form a longi- 

 tudinal ridge in the lateral wall of the fourth ventricle. In 

 mammals these nuclei lie deeper in the substance of the 

 medulla and form the nucleus of the fasciculus solitarius 

 (Sections 84 and 110). In one place only this nucleus in man 

 reaches the ventricular surface, viz., in the fovea inferior and 

 the lateral border of the ala cinerea, which, accordingly, con- 

 tains both visceral motor and visceral sensory centers. This 

 implies that the fovea inferior does not mark the exact site 

 of the embryonic sulcus limitans, but lies somewhat laterally 

 of it. 



56. Somatic sensory column. In mammals, as in the dog- 

 fish, this column of the medulla oblongata contains two clearly 

 separate regions, (1) the general somatic sensory centers and 

 (2) the area acustica (see Section 57). Only the first of these 

 will be considered in this section. 



In either the sheep or the human brain an examination of the 



