128 



THE NERVOUS SYSTEM 



caruleus, beneath which lies the substantia ferruginea, composed of pigmented 

 nerve-cells. 



Beginning at the cerebral aqueduct and extending through both the superior 

 and inferior foveae is a very important groove, the sulcus limitans, which repre- 

 sents the line of separation between the parts derived from the alar plate and 

 those which originate from the basal plate of the embryonic rhombencephalon. 

 Lateral to this sulcus lie the sensory areas of the ventricular floor, including the 

 area acustica, all of which are derived from the alar plate. Medial to this 

 sulcus there is a prominent longitudinal elevation, known as the medial eminence, 

 which includes two structures already described, namely, the facial colliculus 

 and the trigone of the hypoglossal nerve. Beneath the medial part of this 



Tel a chorioidea 



Choriold plexus 



Median aperture of 

 fourth ventricle 



Fig. 90. Dorsal view of human rhombencephalon showing tela chorioidea and chorioid plexus of 



the fourth ventricle. 



trigone lies the nucleus of the hypoglossal nerve and beneath the lateral part is a 

 group of cells designated as the nucleus intercalatus. 



One or two features remain to be mentioned. At the caudal end of the ala 

 cinerea is a narrow translucent obliquely placed ridge of thickened ependyma, 

 known as the funiculus separans. Between this ridge and the clava is a small 

 strip of the ventricular floor, called the area postrema, which on microscopic 

 examination is found to be rich in blood-vessels and neurogliar tissue. 



The roof of the fourth ventricle is formed by the anterior medullary velum, 

 a small part of the white substance of the cerebellum, and by the tela chorioidea 

 lined internally by ependymal epithelium (Fig. 85). Caudal to the cerebellum 

 the true roof of the cavity is very thin and consists only of a layer of ependymal 

 epithelium, which is continuous with that lining the other walls of the ventricle. 



