FOURTH VENTRICLE. 217 



piece is the same as that of the rest of the cerebellum, viz., a central white 

 portion and investing laminae. Here the branching appearance of a tree 

 (arbor vitae) is best seen, in consequence of the lamina? being more divided, 

 and the white central pieces being longer and more ramified. 



The FOURTH VENTRICLE (fossa rhomboidalis) is a space between the 

 cerebellum and the posterior surface of the medulla oblongata and pons 

 (fig. GM). It has the form of a lozenge, with the points placed upwards 

 and downwards. The upper angle reaches as high as the upper border of 

 the pons; and the lower, to a level with the inferior end of the olivary 

 body. Its greatest breadth is opposite the lower edge of the pons; and a 

 transverse line in this situation would divide the hollow into two triangu- 

 lar portions upper and lower. The lower half has been named calamus 

 scriptorius from its resemblance to a writing pen. 



The lateral boundaries are more marked above than below. For about 

 half way down, the cavity is limited on each side by the superior peduncle 

 of the cerebellum (/), which projecting over it forms part of the roof; and 

 along the lower half lies the eminence of the restiform body (fig. 55, 3 ). 



The roof of the space is somewhat arched, and is formed above by the 

 valve of Vieussens (&), and the under part of the vermiform process ; and 

 below, by the reflection of the pia mater from that process to the spinal 

 cord. 



The floor of the ventricle is constituted by the posterior surfaces of the 

 medulla oblongata and pons, and is grayish in color. Along its centre is 

 a median fissure, which ends below, near the point of the calamus, in a 

 minute hole the aperture of the canal of the cord. On each side of the 

 groove is a spindle-shaped elevation, the fasciculus s. eminentia teres (o). 

 This eminence reaches the whole length of the floor, and is pointed and 

 little marked interiorly, where it is covered by gray substance ; but it be- 

 comes whiter and more prominent superiorly, and its widest point is oppo- 

 site the lower border of the pons. 



The outer border of the eminence is limited externally by a slight 

 groove, which points out the position of two small fossae (fovea anterior et 

 posterior). The posterior (r) is near the lower end of the groove ; and 

 the anterior (p) is opposite the crus cerebelli. Above the anterior fossa 

 is a deposit of very dark gray substance, which has a bluish appearance 

 as it is seen through the thin stratum covering it ; ! from it a bluish streak 

 is continued upwards, at the outer edge of the eminentia teres, to the 

 opening in the top of the fourth ventricle. 



Crossing the floor on each side, opposite the lower border of the pons, 

 are some white lines, which vary much in their arrangement (fig. 55) : 

 thev issue from the central median fissure, and enter the auditory nerve 

 (p. 181). 



Besides the objects above mentioned, there are other eminences in the 

 floor of the ventricle indicating the position of the nuclei of origin of cer- 

 tain nerves. 



In the lower half of the space are three slight eminences on each side 

 for the hypo-glossal, vagus, and auditory nerves : that for the hypo-glossal 

 is close to the middle line below, and corresponds with the lower pointed 

 end of the eminentia teres. The other two, outside that eminence, are 

 placed in a line one above another, but separated by a well-marked groove 



1 The term locus caeruleus has been applied to the spot, and the dark vesicular 

 matter in it has been named substantia ferruginea. 



