814 



THE NERVOUS SYSTEM 



Beginning at the calamus scriptorius, the following areas of the floor of the fourth ventricle 

 are usually distinguished (fig. 640) : — 



The area postrema of Retzius is a superficial vascular structure bounded inferiorly by the 

 taenia and overlying the terminal portion of the nucleus of the fasciculus gracilis (clava) and a 

 portion of the nucleus of termination of the vagus nerve. The funiculus separans, a short 

 oblique fold of the floor, composed chiefly of neurogha, separates the area postrema from the 

 ala cinerea {trigonum vagi), which is an oblique, grey-coloured, wing-shaped eminence indicating 

 the middle third of the nucleus of termination (recipient nucleus) of the vagus and glosso- 

 pharyngeal nerves. At the superior extremity of the ala cinerea is a well-marked triangular 

 depression of the limiting sulcus known as the inferior fovea. Mesial to and extending above 

 the ala cinerea is a narrow triangular eminence lying close to the median sulcus, which represents 

 the nucleus of origin of the hypoglossal nerve, the hypoglossal eminence [trigonum n. hypoglossi]. 

 The lateral field of this eminence shows small oblique rugae, giving it a "feathery" appearance, 

 the area plumiformis of Retzius. The nucleus intercalatus of Van Gehuchten is a wedge- 

 shaped portion very slightly demarcated from the hypoglossal eminence, and intercalated 

 between it and the inferior fovea. This nucleus is considered by some observers as an inferior 



Fig. 640. 



-Dorsal Surface of the Brain-stem Showing the Anatomy of the Floor of the 

 Fourth Ventricle. (Modified from Spalteholz.) 



Median sulcus 



Superior fovea ^^ 



Limiting sulcus - 



Medial eminence ^''' / 



Acoustic medullary striae 



Inferior fovea 



Nucleus of fasciculus cuneatus — ' " 



Taenia of fourth ventricle-'' ^-fe^ JV'-' f ^/' ^^ ■■'' 

 Area postrema' jt^* ^ -m < vij 



Nucleus of fasciculus gracilis (clava) 



Posterior median fissure^ 



Aqueduct of cerebrum 



.y ,, Nucleus incertus 



~~ Locus caeruleus 



._\ Eminence of facial 



"T " and abducens 



^.^ Nucleus of coch- 



"■ -'^>!-~- learis (tuberculum 



acusticum) 



^'^'_ Acustic area (nucleus 



/ vestibularis) 



~- Nucleus intercalatus 

 " Hypoglossal eminence (trigone) 

 •■Irigonum vagi (ala cinerea) 



^ Funiculus separans 

 Obex 



medial extension of the nucleus of termination of the vestibular nerve (area acustica), but 

 Streeter, who has made a detailed study of the floor of the fourth ventricle by means of serial 

 sections, doubts that it is a part of this nucleus. It is much more probable that it supplies 

 visceral efferent fibres to the vagus and is thus a continuation of the dorsal efferent nucleus of 

 the vagus. 



Superior to the inferior fovea, and crossing each half of the floor of the fourth ventricle, 

 are the acoustic striae. These are bundles of axones arising in the dorsal nuclei of termination 

 of the cochlear or auditory nerve, whi(;h arc situated in tlic lateral periphery of each restiform 

 body. The bundles cour.se around the dorsal periphery of the upper portion of the restiform 

 body, then across each half of the floor of the ventricle to the median sulcus, in which they 

 suddenly turn ventrally into the substance of the medulla oblongata, and in doing so they cross 

 the mid-line to enter the substance of the opposite side. The striae vary greatly in different 

 individuals, both in the degree of their prominence and their direction. Sometimes no strise 

 are visible from the surface. Frequently a bundle may be discerned which courses obliquely 

 upward and lateralward from tlie median sulcus to disappear in the floor further away from 

 the mid-line and again, a bundle may depart from the transverse course before reaching the 

 median sulcus. Such a bundle ascending is sonu'times called conductor sonorus. The acoustic 

 stria cross the acoustic area. This is the flattened elevation which occupies the whole lateral 

 portion of the intermediate portion of the floor of the ventricle, lateral to the limiting sulcus, 

 and extends into the inferior portion lateral to the inferior fovea. It represents the subjacent 



