THE FOURTH VENTRICLE. 



1099 



The prominence of the eminentia teres is due to the underlying nucleus of the sixth nerve, 

 enclosed by the knee of the facial ; for it, therefore, Streeter proposes the name eminentia abdu- 

 centis. The longitudinal ridge that continues upward and bounds the median fovea, the last 

 cited author interprets as due to a field of gray matter, thin in the vicinity of the abducent 

 eminence and thicker above, to which the name nucleus incertus is applied. Lateral to the 

 nucleus incertus and the facio-abducent eminence, lies the fovea anterior, which elongated and 

 depressed area (nearly 6 mm. long by i mm. wide ) is due to the exit of the root of the fifth 

 nerve ; it may, therefore, be called the fovea trigemini. The median portion of the elevated 

 acoustic area includes the elongated and irregularly lozenge-shaped vestibular area, that 

 measures about 16 mm. in length by 4 mm. in breadth and extends from the fovea anterior 

 (trigemini) to the nucleus gracilis. The lateral part of the area acustica is occupied by the 

 cochlear area, which stretches into the recessus lateralis and overlies the nucleus cochlearis. 



fourth V 



Tela chorioidea 

 and choroid plexus 



White core of 

 cerebelluHi 



Inferior worm 



roof of fourth ventricle is seen 



The Roof of the Fourth Ventricle. — Viewed in median sagittal section (Fig. 

 938), the roof of the fourth ventricle appears as a tent-like structure, whose walls, 

 where they come together, bound a space, the recessus tecti, that penetrates the 

 cerebellar medulla 



between the superior F'g. 950. 



and inferior worm. 

 The upper wall of 

 the tent is formed by 

 the superior med- 

 ullary velum, the 

 triangular sheet of 

 white matter stretch- 

 ing from beneath 

 the quadrigeminal 

 bodies above to the 

 medullary substance 

 of the cerebellum 

 below, and is over- 

 laid by the rudimen- 

 tary cerebellar folia 

 of the Hngula. It 

 must be understood 

 that the ventricular 

 surface of the velum 

 is clothed by the 



ependyma — as are all other parts not only of the fourth ventricle but of all the 

 ventricular cavities. Laterally the superior medullary velum is attached to the 

 superior cerebellar peduncles, which to a limited extent share in closing in this 

 part of the ventricle (Fig. 936). 



The lovi^er half of the roof- comprises two parts, an upper and thicker crescertic 

 plate of white matter, the i?i/erwr med^dlary velum, and a lower and extremely chin 

 membrane, the tela chorioidea. Medially the inferior medullary velum is attached 

 for some distance to the front and lower surface of the nodule, which it excludes, 

 strictly regarded, from the ventricle, whilst laterally the velum is prolonged to the 

 flocculus, its fibres becoming continuous with the white core of this subdivision of 

 the cerebellum. The nervous constituents of the velum extend only as far as its 

 crescentic lower border, beyond which the roof of the ventricle, in a morphological 

 sense, is formed by the ependymal layer alone. This, however, is supported by a 

 backing of pial tissue, which, in conjunction with the ependyma, forms the tela 

 chorioidea. On nearing the lower angle of the ventricle, the roof presents a trian- 

 gular thickening, the obex, that closes the cleft between the clavae and lies behind 

 (above) the nib of the calamus scriptorius. 



On each side the obex, which consists of a layer of white matter fused with the 

 underlying ependyma, is continuous with the slightly thickened margin of the roof, 

 the taenia ventriculi, whose line of attachment passes from the clava upward and 

 outward over the cuneate tubercle of the medulla and the restiform body and, farther 

 upward, runs obliquely across the dorsal surface of this peduncle to close in the lateral 



Dorsal portion of preparation shown in Fig. 948 ; 



from below. 



