710 THE yKRVOUS SYSTEM 



wards until the quadrigemiiial bodies and the latiTal parts of tlie crura cerebri are fully exposed. 

 The valve of A^ieusseus and the slender pair of iourth nervi-s which spiin^^ from its upper jiart 

 are verj' liable to injury at this stage. In conducting this dissection, the >tuileut will notice a pair 

 of broad white bands, the superior peduncles oi' the cerebellum, converging towards the lower 

 pair of quadrigeminal bodies, and a thin white lamina, the valve of A^ieussens, filling tlie inter- 

 space between them. The mesencephalon is now fully exposed, and its Iree surface should fir.^t 

 be examined. Directions will be subsequently given lor the examination of its internal structure. 



The Mesencephalon 



The aqueduct of Sylvius is a narrow passage, a little more than half an inch 

 long, Avhich unites the cavities of the third and fourth ventricles: hence it is 

 sometimes called iter e tertio ad quartum ventriculum. It is roofed by a plate 

 of grey matter, the lamina quadrigemina, which is surmounted by tAVO jDairs of 

 elevations, the corpora quadrigemina. Its floor is formed by the tegmenta of 

 the crura cerebri. Its out line in transverse section varies in different parts of its 

 course. Thus, at its commencement, under the posterior commissure, it has the 

 form of an isosceles triangle with the apex directed downwards. Below the anterior 

 quadrigeminal bodies it is heart-shaped, then somewhat shield-shaped. Lower 

 down, on the level of the posterior corpora Cjuadrigemina, it has the form of an 

 ovate leaf with the stalk directed downwards; and finally, at its junction with tlie 

 fourth ventricle, it is T-shaped in section (fig. 444). 



The grey matter of the aqueduct is a thick stratum which surrounds that canal, and is con- 

 tinuous with the grey matter of the floor of the fourth ventricle. It is bounded below by the 

 formatio reticularis, and by the posterior longitudinal bundle (fig. 424). It is continuous above 

 with the grey matter of the lamina quadrigemina. Groups of cells are embedded in this grej' 

 substance ; the most important of these is a long column of nerve-cells which is placed near the 

 middle line beneath the aqueduct close to the formatio reticularis (fig. 424). This is the con- 

 tinuous nucleus of the third and fourth nerves. It commences near the junction of the tliird 

 ventricle and the aqueduct, and extends for nearly the whole length of the latter. It is on a line 

 witli the nuclei of two nen'es (sixth and twelfth) which arise from the floor of the fourth ventricle. 

 The nucleus of the descending root of the fifth nerve is a group of cells in the region of the 

 upper quadrigeminal bodies, situated above and external to the nucleus of the third nerve ; it is 

 limited on its outer side by the white fibres of the nerve-root itself 



The formatio reticularis will be described with the anatomy of the medulla and pons. 



The CORPORA QUADRIGEMINA are four greyish elevations which surmount 

 the lamina (juadrigemina. The superior or anterior pair are termed the nates ; the 

 inferior or posterior pair, the testes. Tlie corpora quadrigemina arc marked oft' 

 by a cruciform groove, the transverse limb of which is the more distinct. The 

 portion of the vertical groove which lies between the nates is also well marked, l)Ut 

 the groove between the testes is shallow and is obscured by a rounded bundle of 

 fibres, the frenulum veli, Avhich passes downwards and spreads out on the valve 

 of Vieussens, immediately below the testes. The corpora quadrigemina consist of 

 grey matter overlaid by a thin superficial stratum of white fibres. 



The nates are broader and darker in colour than the testes, and form a pair of 

 ovoid elevations with their long axes directed ujiwards and outwards; this direction 

 is continited into the corresponding brachia. 



The brachia of the nates, or superior brachia, are sliglitly-raised white bands 

 which pass on each side from the nates in the intervals between the pulvinar of the 

 optic thalami and the corpora geniculata interna. They are sharply marked off 

 from the latter, but less distinctly from the thalami. Each brachium is contimied 

 below the corpus geniculatum externum of its own side directly into the optic 

 tract. 



The testes are lighter in colour, and form more ])ronnimced elevations tlian the 

 nates. Each testis is a somewhat pear-shaped body, the narrow end of which is 

 continued into the lirachium of its own side. 



The brachia of the testes, or inferior brachia, are narrower and more promi- 

 nent than the brachia of the nates, and run iiaralld to the latter. Each brachium 

 ]>asses below the corpus geniculatum internum of its own side and disap])ears from 

 the surface. 



The corpus geniculatum internum is an oval elevation which is jdaced behind 



