THE FOURTH VENTRICLE 



813 



attachment to the surface of the medulla it is thickened, and in sections this por- 

 tion bears the name ligula (tcenia ventriculi quarti). The thickest portion spans 

 the tip of the calamus scriptorius and is termed the obex. ^ The width of the ven- 

 tricular cavity is extended laterally from its widest part into the lateral recesses., 

 narrow pockets on each side and around the upper parts of the restiform bodies. 

 In the mid-Une of the lower part of the chorioid tela there is a more or less well- 

 marked opening, the foramen of Magendie (medial aperture of the fourth ventricle), 

 which is a lymph-channel connecting the cavity of the ventricle with the subarach- 

 noid space. There is a similar opening from each lateral recess (lateral apertures 

 of Key and Retzius) . 



The chorioid plexuses of the fourth ventricle consist of highly vascular, lobular, 

 villus-like processes of the ventricular lining (and pia-mater) of the chorioid tela. 

 They are reddish in the fresh specimen, and the epithelial lining of the ventricle is 

 closely adapted to the unevennesses of their surfaces. From below they run as 



Fig. 639. — Diagram of the Roof and Lateral Boundaries of the Fourth Ventricle. 

 The trochlear nerve should be shown emerging from the lateral boundary of the frenulum veU. 



luferior quadrigeminate body 

 Trochlear nervo 



Acterior medullary velum 

 Biachium conjunctivTim 



Brachium of pons 



Restiform body 



Ligula tasnia 

 Chorioid tela of 

 fourth ventricle 



Cuneate tubercle 



Clava 



Tubercle of Rolando 



Frenulum veli 

 Lateral lemniscus 



Lingula of vernis 



Fourth ventricle 



Posterior medullary velum 

 Chorioid plexus 



Foramen of Magendie 



two parallel masses on either side of the mid-line, which become united above, 

 and then are separated again into two lateral processes which bend at right angles 

 and project into the lateral recesses. Portions frequently protrude through the 

 three openings of the ventricle into the subarachnoid space. 



The floor of the fourth ventricle [fossa rhomboidea] (fig. 640). — This is thrown 

 into eminences and depressions indicative of the internal structures of the 

 brain-stem subjacent to it. Its inferior portion is the dorsal surface of the upper 

 portion of the medulla oblongata; its intermediate portion is the dorsal surface 

 of the pons region, while its superior portion belongs to the isthmus of the 

 rhombencephalon. Its triangular lower extremity terminates as the opening of 

 the central canal of the spinal cord. This portion is deepened at the obex and 

 shows furrows which point downward and converge medialward, giving the 

 appearance known as the calamus scriptorius. The mid-line of the floor is sharply 

 distinguished by the well-marked median sulcus, which becomes shallower above 

 than below. In the tip of the calamus scriptorius, immediately anterior to the 

 obex, the median sulcus deepens to become continuous into the central canal. 

 This terminal depression is known as the ventricle of Arantius. Throughout the 

 length of the floor on either side of the median sulcus is a continuous ridge, the 

 medial eminence, which is bounded laterally by the limiting sulcus. Underlying 

 the floor of the ventricle is a layer of grey substance of varying thickness, which is 

 continuous with that surrounding the central canal of the cord. The medial 

 eminence is subdivided into portions of unequal width and elevation, and the 

 limiting sulcus accordingly shows fovese of different depths. 



