1098 HUMAN ANATOMY. 



superior fovea, the ventricular floor presents a slightly sunken field, the locus 

 cceruleus, which extends upward to the Sylvian aqueduct and in fresh preparations 

 possesses a bluish gray tint in consequence of the deeply pigmented cells of the 

 underlying substantia ferruginea (page 1081) showing through the ependymal layer. 



The accurate description of the surface markings of the ventricular floor given by Retzius, 1 

 has been supplemented by Streeter's 2 careful study of the relation of these details to the under- 

 lying structures. The most important results of these observations, which have materially 

 advanced our understanding of this important part of the brain-stem, may here find mention. 



The trigonum hypoglossi is seen, especially when examined under fluid with a hand-lens, 

 to include two subdivisions, a narrow median and a broader lateral. The first of these is con- 

 vex, about 5 mm. long by i mm. wide, and corresponds to the rounded upper end of the nucleus 

 of the twelfth nerve ; it is, therefore, appropriately called the eminentia hypoglossi (Streeter). 

 The entire hypoglossal nucleus, however, is of much larger size (about 12 mm. long by 2 mm. 

 wide) and extends some 5 mm. below the tip of the calamus scriptorius, ventral (anterior) to the 



FIG. 949. 



Colliculus inferior IV. nerve 



>* 



Superior cerebellar peduncle ^_^ \^^ - ^S 



Superior fovea -. __ Jfcl( ^^"^ ^^ Area n. abducentis 



Trigonum acustici ^-^*"^^^t"^^B HC^^Funiculus solitarius 



Trigonum hypoglossi ^^^^^P^^^^^f^A """-Area n. vagi 



Floor of the fourth ventricle; areas corresponding to nuclei of nerves are shown on right half of 



figure. X J. (Streetft.) 



vagus nucleus and nucleus gracilis. Lying immediately above the hypoglossal eminence is a 

 second and somewhat less pronounced elevation, formed by the nucleus funiculi teretis and meas- 

 uring nearly 6 mm. in length by i mm. in breadth. Lateral to these two median elevations and 

 limited externally by the ala cinerea, lies a wedge-shaped field that is insinuated between the 

 hypoglossal eminence and the vagal trigone. It stretches from the acoustic striae above to the 

 nib of the calamus scriptorius below. This field, named the area plmnifonnis by Retzius on 

 account of its feather-like markings, is regarded by Streeter as corresponding to a group of cells, 

 the nucleus intercalatus, that occupies a superficial position in the ventricular floor and partly 

 overlies the hypoglossal nucleus. 



The fovea vagi (ala cinerea), which lies lateral to the nucleus intercalatus, corresponds to 

 the middle and superficial third of the vago-glosso-pharyngeal nucleus, the entire extent of tin- 

 latter including a tract measuring about 13 mm. in length by 2 mm. in breadth, that stretches 

 rrom beneath the vestibular nucleus above to over 2 mm. beyond the inferior angle of the 

 ventricle. The lower third of the area of the va.nus nucleus is partly within the ventricle; 

 immediately above the obex this intraventricular portion is covered by a layer of loose vascular 

 tissue and appears as an upwardly diverging pointed field, area postrema of Retzius. This is 

 separated from the ala cinerea by a translucent ridge, the funiculus separans, composed of 

 thickened ependymal neiiro^lia (Streeter). . 



1 Das Menschenhirn, 1896. 



1 Amer. Journal of Anat. Vol. II, 1903. 



