THE FOURTH VENTRICLE. 



1097 



with the dehcate roof-membrane, known as the tela chorioidea. Toward its upper 

 end the longitudinal furrow presents a second expansion, the fossa mediana. The 

 acoustic striae vary greatly in distinctness and arrangement, sometimes appearing as 

 well-marked bands that cross the ventricular floor with litde divergence, or they may 

 constitute a fan-shaped group in which the strands may be irregularly disposed or 

 even overlap ; in other cases they may be much less distinct on one side, or so feebly 

 marked on both as to be unrecognizable. Quite frequently one band diverges from 

 the others and crosses the floor obliquely upward and outward. This strand, spe- 

 cially designated as the conductor sonorus, is seldom equally distinct on the two 

 sides, being usually better seen on the left. 



The inferior division of the ventricular floor, that lying below the acoustic 

 striae, presents three general fields of triangular outline. The one next the median 

 groove, with its base above and its apex directed towards the lower angle of the 

 ventricle, which it almost reaches, is the trigonum hypoglossi, so called from the 

 fact that it partly overlies the nucleus of the twelfth nerve. Lateral from the last 



Fig. 948, 



Corpora quadrigemina 

 Sylvian aqueduct 



cerebellar peduncles 



- Eminentia teres 



Trigonum 

 hypoglossi 



Trigonum vagi 



White core of 

 cerebellum 



Trigonum acustici 



' Funiculus gracilis 

 Floor of fourth ventricle exposed after removal of its roof by frontal section. 



named area is a somewhat depressed triangular field of darker color, the apex of which 

 is placed above, near the acoustic striae, and the base below ; this field is known as the 

 ala cinerea, from the dark tint imparted to it by the pigmented cells lying beneath, 

 and as the trigonum vagi, in recognition of the subjacent glosso-pharyngeo-vagus 

 nucleus. The remainder of the inferior division of the ventricular floor includes an 

 elevated triangular field, the trigonum acustici, that is part of the larger tract, the 

 area acustica, which occupies not only the lateral angle of the rhomboidal fossa, 

 where it is crossed by the acoustic striae, but also the adjacent portion of the superior 

 division of the ventricular floor. Laterally, the acoustic area presents a distinct 

 elevation, the tuberculum acusticum, which, together with the adjacent part of 

 the trigonum acustici, is related to the nuclei of the cochlear nerve ; the more median 

 portion of the acoustic area, on the other hand, belongs to the vestibular division. 



The superior division of the ventricular floor, above the acoustic striae, is 

 marked on each side of the median groove by a prominent elevation, the eminentia 

 teres, which below is continuous with the trigonum hypoglossi and above narrows and 

 fades away towards the floor of the Sylvian aqueduct. Laterally the eminence is 

 bounded by a depressed area, the fovea superior, which is the expanded upper part 

 of a second longitudinal furrow, the sulcus lateralis, that defines the outer limit oi 

 the eminentia teres and below is continued into the depressed trigonum x'agi, to which 

 the name, fovea inferior, is sometimes applied. Above and to the outer side of the 



