FOURTH VEXTRICLE—POXS VAROLII 719 



The floor of the fourth ventricle is a depressed area, Avhieli is rhomboidal in 

 outline, the longer diainctfi' of tlic rhomboid Ijeing directed upwards and forwards 

 and marked by a very distinct tissure, the sulcus longitudinalis medianus, wliich 

 is continuous below with the central canal of the spinal cord. The ventricular floor 

 is bounded laterally hy the clava? and corpora restiformia below, by the middle 

 peduncles of the cerebellum in the middle, and by the superior peduncles of the 

 cerel)ellum above. Under cover of the obex, where the fourth ventricle is about to 

 join the central canal of the cord, a small amjjulla is formed, termed the ventricle 

 of Arantius, The widest part of the floor corresponds to the lateral recesses of the 

 ventricle. Close Ixdnnd the lateral recesses, the floor is crossed by some whit(^ 

 lines, the striae meduUares or striae acusticae, which emerge from the sulcus 

 longitudinalis medianus, and pass outwards to the auditory nerve. The stria? 

 diviile the flo(jr into two ai)])roximattdy equal parts, an inferior and a superior. 



The inferior moiety of the floor is indented b}' an angular groove, the 

 fovea inferior ijr posterior, wliieh marks off a depressed triangular area of a grey 

 colour, the ala cinerea. The latter corresponds in position to the subjacent nuclei 

 of tlie vagus and glosso-}»harvngeal nerves, and is therefore also termed the trigo- 

 num vagi. The apex of the posterior fovea is directed forwards. It nearly touches 

 the striffi acusticic, and thus two other triangular areas, both of a Avhitish colour, 

 are marked off in the inferior moiety of the floor. The internal of these areas is 

 called the trigonum hypoglossi, since it corresponds in po.^ition to the nucleus of 

 the livpoglossal nerve. It is l)Ounded by the inferior fovea, the striae acustica?, 

 and the sulcus longitudinalis medianus. The outer triangle is bounded by the 

 inferior fovea, by the striae acusticse, and by the clavse. It forms the inferior part 

 of a prominence or swelling which extends into the pontine portion of the floor. 

 This prominence is called the tuberculum acusticum. It is well marked in the 

 brain of the infant. 



The superior moiety of the floor is completely covered by a thin layer of white 

 substance. It presents, on each side of the sulcus longitudinalis medianus, a semi- 

 cylindrical elevation about five millimetres in length, which is called theeminentia 

 teres. It is caused by an underlying bundle of fibres, the fasciculus teres, 

 wiiich is now known to be a part of the intrapontine course of the fibres of the 

 facial nerve, though possibly it may contain fibres from other sources. External 

 to the eminentia teres is a depressed area, the fovea superior, which is occasion- 

 ally crossed by a whitish band of fibres, the conductor sonorus. The conductor 

 sonorus meets the stria medullaris of its own side, at the sulcus longitudinalis 

 medianus, at an acute angle. Immediately in front of the fovea superior is a Ijluish 

 spot, the locus caeruleus, which is caused by the substantia ferruginea showing 

 through the white covering of the floor. The substantia ferruginea is an aggre- 

 gation of darkly-pigmented cells, which is very conspicuous in transverse sections 

 through the upper part of the floor of the fourth ventricle and through the lower 

 part of the aqueduct of Sylvius. 



The PONS VAROLII is that portion of the epencephalon which lies in front of the 

 proximal part of the fourth ventricle. As seen from the base of the brain, it forms 

 a considerable prominence, which narrows on each side as it passes into the middle 

 cerebral peduncles. At its lower border the pyramidal bodies are seen disappear- 

 ing into its substance, while at its upper border the crvu'a cerebri are seen emerging. 

 It rests on the body of the si)henoid Ijone, reaching upwards as far as the margin 

 I >f the dorsum sellse. It ]>resents a ventral and a dorsal surface, and a superior and an 

 inferior 1 )order. Lateral surfaces have also been described, l)ut these are determined 

 artificially l)y dividing the pons proper from the middle cerebellar peduncles. The 

 boundary between the pons and the middle cerebellar peduncles on each side is 

 arbitrarily mapped out by a line drawn from the exit of the trigeminal nerve to the 

 superficial origin of the facial. (Henle. ) 



The dorsal surface has been described in connection with the floor of the fourth 

 ventricle. 



The ventral surface is broad and prominent, strongly convex from side to side, 

 and slightly convex from l)efore Itackwards. It presents a transverse striation, cor- 

 responding to the bundles of commissural fibres passing from hemisphere to 



