AREAS OF THE MEDULLA OBLONGATA 869 



the striae obliquae (conductor sonorus). This irregularity of the acoustic striae 

 has led to another mode of division of th/ "floor" for descriptive purposes each 

 side to be divided into a median and a lateral area, indicated by a more or less 

 well-marked groove, the lateral furrow (sulcus limitans), connecting the superior 

 and inferior (ala cinerea) fovese. This groove probably corresponds to one of 

 the interzonal sulci of the embryonic tube, and in a gross way it separates the motor 

 and sensor fields of the "floor." The median area is usually a continuous ridge 

 which is quite accentuated in the cephalic division as the eminentia abducentis 

 (e. teres), while caudad it becomes narrowed as it approaches the closed part of 

 the medulla oblongata. The convergence of the median and lateral furrows at the 

 caudal apex of the rhomboidal fossa gives the appearance of the point of an ancient 

 writing reed or quill pen; hence the term calamus scriptorius. 



In the caudal quarter triangle a middle area is occupied by an elongated tri- 

 angular field whose depressed apex is directed frontad. A slight oblique ridge, 

 the funiculus separans, composed chiefly of neuroglia, separates the area postrema 

 caudally from the trigonum vagi or 'ala cinerea of a pronounced grayish color. The 

 whole depression has been termed the fovea inferior. Mesally lies a narrow 

 triangular field with its apex directed caudad and with slightly raised surface 

 the trigonum hypoglossi. This area is resolved into two fields by a single or double 

 formation of oblique rugse affording a "feathered" appearance to the lateral field, 

 the area plumiformis. Laterad of the trigonum vagi lies the caudal portion of the 

 lateral area of the "floor," also called (in part) the area vestibularis (area acustica) 

 and crossed over its middle by the striae acusticae when these are present. The 

 area vestibularis is an irregularly triangular raised surface with its convex base 

 toward the median line, and extending laterally to the attachment of the tela 

 choroidea and into the lateral recess. In the fetus and in certain lower vertebrates 

 the area is more prominent and is designated the tuberculum acusticum s. vestibu- 

 laris. 



The "frontal" division of the floor or triangular quarter-field is marked by a 

 depression at about its middle, the superior fovea, from which the slight "lateral 

 furrow" runs caudad, and but for the intervention of the strire would reach the 

 inferior fovea. Cephalad of the superior fovea, and continuing some distance 

 along the aqueduct, is the locus caeruleus, which owes its color to the refraction 

 of the pigmented cells, the substantia ferruginea, by the milky-white ependyma. At 

 this altitude, the medial elevation between the superior fovea and the median 

 sulcus is accentuated into a fairly pronounced eminence, the eminentia abducentis 

 (eminentia teres], overlying the nucleus of the abducent nerve and the genu of 

 the root of the facial nerve. The portion of the median sulcus intervening between 

 the eminentia abducentia is correspondingly depressed to form the fovea mediana. 



The ventricular features enumerated above correspond in a crude w r ay to the 

 deep structures of the pons oblongata, and most of the cranial nerve nuclei are 

 held in a rhomboidal frame formed by the superior and inferior peduncles. The 

 surface markings are only imperfect replicas of the subjacent structures; the various 

 cell nests overlap each other more or less and their relations can best be studied in 

 the projection drawing in Fig. 634. 



Membranous Portion of the "Roof" of the Fourth Ventricle. The caudal exten- 

 sion of the hypertrophied cerebellum hides from view the whole of the rhomboidal 

 fossa, but this structure, as before stated, forms but a part of the actual dorsal 

 wall or "roof." This includes the converging superior peduncles, the velum medul- 

 lare anterius intervening between these, the fastigium of the cerebellum, the velum 

 medullare posterius, and the tela choroidea ventriculi quarti. 



The velum medullare posterius is a thin and narrow lamina of white substance 

 continued laterad as the flocculi of the cerebellum. At its caudal edge, i. e., 

 where nerve tissue ceases, the ependymal or ventricular lining epithelium and the 



