THE BRAIN 121 



Structures on the floor. 

 Lower half. 

 Median groove. 



Inferior fovea. A triangular pit on each side of the 

 median groove near the striae acusticae, the apex being 

 directed upwards. From the base two grooves pass down- 

 wards, one towards the calamus scriptorius, and the 

 other towards the lateral boundary. Three areas are 

 thus marked off in each lateral half of the medullary 

 portion : 



1. Trigonum hypoglossi, next to median groove, subjacent 



to which is the hypoglossal nucleus. 



2. Trigonum vagi, included between the two grooves 



passing from the fovea. Contains nucleus of vagus 

 below, and nucleus of glosso-pharyngeal above. 



3. Trigonum acustici (lower half), most external, con- 



tains chief part of auditory nucleus, forming part 

 of acoustic tubercle. 



Upper half. 

 Median groove. 



Superior fovea. A triangular depression in a line with 

 the inferior fovea, dividing off each lateral half into three 

 areas, viz. : 



1. Fasciculus teres, most internal, here raised into the 



eminentia teres, and continuous with the trigonum 

 hypoglossi below. 



2. Locus cceruleus, a depression extending from the 



superior fovea to the upper end of the ventricle, 

 beneath which is some pigment, the substantia 

 nigra. 



3. Tuberculum acusticum (upper half). 



Issuing from the median groove, and passing outwards on 

 each side over the middle of the tuberculum acusticum are 

 some white lines, the stria acusticee. 



Lateral recess. This is a lateral extension between the cere- 

 bellum and the medulla of the cavity of the ventricle at its 

 widest part. 



The roof : in the lower half is formed by pia mater, lineti 

 with epithelium. It has three apertures, one on each side in 

 the lateral recess, and the third at the apex of the calamus 

 scriptorius, the foramen of Majendie, by which the 4th ventricle 

 communicates with the subarachnoid space. 



On each side where the pia mater is attached laterally to 



