52 LABORATORY OUTLINE OF NEUROLOGY 



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 stria medul- 

 lares acustici. 



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

 visceral sensory nuclei of the cranial nerves form a longitudinal 

 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, contains 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. So?natic sensory column. — In mammals, as in the dogfish, 

 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 

 spinal cord where it joins the medulla oblongata will reveal on 

 the dorsal surface the fasciculus gracilis and the fasciculus cune- 

 atus. These are composed chiefly of fiber tracts of the spinal 

 proprioceptive system and they enter enlargements lying later- 

 ally of the lower end of the fourth ventricle (calamus scriptorius), 

 known respectively as the clava, or nucleus of the fasciculus 

 gracilis, and the tuberculum cuneatum, or nucleus of the fascicu- 

 lus cuneatus. Laterally of the tuberculum cuneatum is the 

 tuberculum cinereum, a longitudinal ridge formed by the spinal 



