1378 A MANUAL OF ANATOMY 



calamus scriptorius, and the outer groove passes downwards and 

 outwards towards the lateral boundary. Between these two 

 diverging grooves there is a triangular area, called the trigonum 

 vagi. Its apex is at the fovea inferior, and its base is directed 

 downwards and outwards. It has a dark colour, and from this 

 circumstance it is known as the ala cinerea. Subjacent to it there is 

 the dorsal nucleus of the vagus and glosso-pharyngeal nerves. 



A second triangular area, called the trigonum hypoglossi, is 

 situated between the median longitudinal groove and the inner of 

 the two grooves prolonged from the angles of the base of the fovea 

 inferior. Its base is directed upwards towards the striae acusticse, 

 and its apex downwards towards the point of the calamus scrip- 

 torius. The area is slightly elevated, and is associated with the 

 lower part of the eminentia teres. Subjacent to the inner part of 

 this area is the upper part of the nucleus of the hypoglossal nerve. 



A third triangular area, called the trigonum acusticum, is situated 

 between the lateral boundary of the floor and the outer of the two 

 grooves prolonged from the angles of the base of the fovea inferior. 

 Its base, like that of the trigonum hypoglossi, is directed upwards, 

 and is continued into an eminence, over which the striae acusticae 

 pass. This eminence is known as the tuberculum acusticum. 

 Subjacent to the trigonum acusticum and tuberculum acusticum 

 there is the dorsal or chief terminal nucleus of the vestibular divi- 

 sion of the auditory nerve. 



Upper or Pontine Part.— The pontine part of the floor is formed 

 by the dorsal surface of the pons Varolii between the converging 

 superior peduncles of the cerebellum, which constitute its lateral 

 boundaries. Its upper somewhat tapering end adjoins the lower 

 end of the aqueduct of Sylvius. Like the bulbar part, it is traversed 

 in the median line by a longitudinal groove, already referred to, 

 which subdivides it into two symmetrical halves. 



Immediately above the striae acusticae, and in line with the fovea 

 inferior, there is a slight depression, called the fovea superior, the 

 two foveae being separated from each other by the tuberculum 

 acusticum . Between the fovea superior and the median longitudinal 

 groove there is a well-marked prominence, called the eminentia 

 teres or medialis. Subjacent to this eminence, immediately above 

 the striae acusticae, is the abducent nucleus, or nucleus of the sixth 

 cranial nerve. The eminence is continued downwards into the 

 trigonum hypoglossi, and superiorly it extends towards the lower 

 end of the aqueduct of Sylvius. Extending upwards from the 

 fovea superior towards the region of the lower end of the aqueduct 

 of Sylvius there is a slight depression, known as the locus cseruleus, 

 which has a dark grey or somewhat blue colour. This colour is due 

 to a subjacent group of deeply pigmented nerve-cells, known as 

 the substantia ferruginea. This group may belong to the chief 

 motor nucleus of the fifth cranial nerve, or it may be a terminal 

 nucleus for some of the sensory fibres of that nerve. 



Dorsal or Posterior Wall of Fourth Ventricle.— This wall is 



