THE FOURTH VENTRICLE 



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part is triangular, with its apex directed rostrally and its base along an imagin- 

 ary line through the superior foveae. The inferior part is also triangular, but 

 with its apex directed caudally and its base at the level of the horizontal por- 

 tions of the taeniae of the ventricle. Between these two triangular portions is 

 the intermediate part of the fossa, which is prolonged outward into the lateral 

 recesses. The floor is covered with a thin lamina of gray matter continuous 

 with that which lines the central canal and cerebral aqueduct. Crossing the 

 fossa transversely in its intermediate portion are several strands of fibers known 

 as the stria medullares acustica. These are subject to considerable variation in 

 different specimens. Springing from the dorsal cochlear nuclei they wind 

 around the restiform body in the lateral recess and run transversely across the 

 fossa to disappear in the median sulcus. 



The inferior portion of the fossa bears some resemblance to the point of a 

 pen and has been called the calamus scriptorius. It belongs to the medulla 

 oblongata. In this part of the fossa there is on either side a small depression, 

 the inferior fovea, shaped like an arrow-head, the point of which is directed toward 

 the striae medullares. From the basal angles of this triangle run diverging sulci: 

 a medial groove toward the opening of the central canal and a lateral groove 

 more nearly parallel to the median sulcus. By these sulci the inferior portion 

 of the fossa is divided into three triangular areas. Of these the most medial 

 is called the trigone of the hypoglossal nerve or trigonum nervi hypoglossi. Be- 

 neath the medial part of this slightly elevated area is located the nucleus of the 

 hypoglossal nerve. The area between the two sulci, which diverge from the 

 fovea inferior, is the ala cinerea or triangle of the vagus nerve. Both names 

 are appropriate, the one, because of its gray color, and the other, because a 

 nucleus of the vagus nerve lies subjacent to it. The third triangular field, 

 placed more laterally, forms a part of the area acustica. 



The area acustica is, however, not restricted to the inferior portion of the 

 fossa, but extends into the intermediate part as well. Here it forms a prominent 

 elevation over which the striae medullares run. Subjacent to this area lie the 

 nuclei of the vestibular nerve. A part of the acoustic area and all of the ven- 

 tricular floor rostral to it belong to the pons. 



Rostral to the striae medullares there may be seen a shallow depression, 

 the fovea superior, medial to which there is a rounded elevation, the facial 

 colliculus. Under cover of this eminence the fibers of the facial nerve bend 

 around the abducens nucleus. Extending from the fovea superior to the 

 cerebral aqueduct is a shallow groove, usually faint blue in color, the locus 



