206 TOPOGRAPHICAL ANATOMY OF THE 



a continuity with the white matter of the vermis comparable to that of 

 the anterior velum — to the flocculus. The velum, however, soon loses 

 its nervous elements and gives place to a very thin translucent membrane 

 composed of pia mater lined with ependyma. This completes the 

 posterior part of the roof of the ventricle, and is attached to the margins 

 of this cavity. 



TIlc Fourtk Ventricle (Ventriculus quartus). — The fourth ventricle 

 represents the cavity of the original vesicle from which the hind-brain 

 developed. Consequently it is in communication with the central canal 

 of the spinal cord at one end and with the cavity of the mid-brain 

 (aquseductus cerebri) at the other. The opening of the central canal 

 into the ventricle has already been commented upon; and, if the 

 anterior medullary velum be raised, the communication with the 

 aqueduct — and, through this with the third ventricle — will be revealed. 



In outline the cavity is rhomboidal ; hence the name fossa 

 rhomboidea^ as applied to its floor. From the narrow posterior end — 

 calamus scriptoriiis" — it widens to the level of the place where the 

 restiform bodies turn up into the cerebellum. Here the transverse 

 diameter is materially increased by the presence of lateral recesses 

 (recessus laterales), which curve round the posterior aspect of the 

 restiform bodies. In front of the recesses the ventricle narrows to the 

 entrance of the aqueduct of the mid-brain. 



The roof or dorsal boundary of the ventricle is formed by the two 

 medullary vela and the thin pial continuation of the posterior velum. 

 Seen in sagittal section, it is somewhat tent-like, being prolonged into 

 a recess between the vermis attachments of the two vela. The roof, 

 moreover, is rendered irregular on a level with the lateral recesses by an 

 invagination produced by the choroid plexus (plexus chorioideus 

 ventriculi quarti) of the choroid tela ^ (tela chorioidea ventriculi quarti) 

 of pia mater. That part of the plexus that is related to the lateral 

 recess is conspicuous in the intact brain as a vascular tuft occupying an 

 angular depression formed by the cerebellum and the medulla 

 oblongata. 



The membranous roof is defective at the extremity of the lateral 

 recess, where a small, irregular opening, the lateral aperture (apertura 

 lateralis ventriculi quarti), places the cavity of the ventricle in com- 

 munication with the subarachnoid space. At the extreme posterior end 

 of the ventricle the roof forms a small blind recess that overlies the 

 entrance to the central canal of the spinal cord. 



1 Fossa [L.], a ditch, a trench, a fosse. povSoeidris (rhombceides) [Gr.], shaped 

 like a rhomb. 



2 Calamus [L.], a reed, a reed-pen. Scriptor [L.], a writer. 



3 Tela [L.J, a web. 



