584 THE NEEVOUS SYSTEM. 



into the interpeduncular fossa, and from it emerge the fila of the oculo-motor 

 nerve. It is termed, therefore, the sulcus n. oculomotor!!. The lateral groove, which 

 is placed on the lateral aspect of the mesencephalon, is called the sulcus lateralis 

 [mesencephali]. Its lower end becomes continuous with the furrow between the 

 brachium pontis and brachium conjunctivum of the cerebellum. 



A close inspection of the lateral surface of the tegmental part of the pedunculi 

 cerebri, below the level of the brachia, will reveal some faintly-marked bundles 

 of fibres curving obliquely upwards and backwards to reach the inferior colliculus 

 (Fig. 517, p. 583). These are fibres of the lateral lemniscus, coming to the surface 

 at the sulcus lateralis and sweeping over the subjacent brachium conjunctivum to 

 gain the inferior colliculus, inferior brachium, and medial geniculate body. 



INTERNAL STRUCTURE OF THE MESENCEPHALON. 



"When transverse sections are made through the mesencephalon the aquseductus 

 cerebri is seen to be surrounded by a thick layer of gray matter, which receives the 

 name of the stratum griseum centrale or the central gray matter of the aqueduct. On 

 the dorsal aspect of this gray matter the corpora quadrigemina form a layer which 



separates it from the surface, and 

 to which the term, lamina, quadri- 

 gemina is applied. On the anterior 

 and lateral aspects of the central 

 gray mattQT are the tegmental por- 



LATERAL. fr^ Q f ^ Cerebrai pgdundCS J 



whilst, intervening between each of 

 the tegmenta and the corresponding 

 basis pedunculi, there is a conspicu- 

 ous mass of dark pigmented matter, 

 termed the substantia nigra. 



OCULOMOTOR Aquaeductus Cerebri and 



Stratum Griseum Centrale. The 



FIG. 518. DIAGRAMMATIC VIEW OF THE CUT SURFACE OF aqueduct is the canal which leads 

 A TRANSVERSE SECTION THROUGH THE SUPERIOR PART f ^. n ,1 *. i i , 



OF THE MESENCEPHALON. from the f urth ventricle below, up- 



wards through the mesencephalon, 



to the third ventricle above. It is not quite three-quarters of an inch in length, 

 and it lies much nearer the dorsal than the ventral surface of the mesencephalon. 

 When examined in transverse section, it presents a triangular outline as it passes 

 into the fourth ventricle and a T-shaped outline close to the third ventricle. In 

 the intermediate part of its course it assumes different outlines, and not always 

 the same form at the same level in different specimens. 



The aqueduct is lined with ciliated epithelium, and outside this is the thick layer 

 of central gray matter, which is directly continuous below with the gray matter 

 spread out on the floor of the fourth ventricle, and above with gray matter on 

 the floor and sides of the third ventricle. Scattered more or less irregularly 

 throughout the central gray matter are numerous nerve-cells of varying forms and 

 sizes, whilst in addition to these there are three definite collections or clusters of- 

 cells, which constitute the nuclei of origin of the trochlear nerve, the oculomotor 

 nerve, and the mesencephalic root of the trigeminal nerve. The position and 

 relations of these will be given at a later stage. 



Substantia Nigra. When seen in transverse section, the substantia nigra 

 presents a semilunar outline. It consists of a mass of gray matter, in the midst of j 

 which are large numbers of deeply pigmented nerve-cells. It is only when this 

 substance is examined in bulk that it appears dark; in thin sections it does not-/ 

 differ much in colour from ordinary gray matter, although, under the microscope, jj 

 the brown -coloured cells stand out very conspicuously, even under low powers.- 

 The substantia nigra is disposed in the form of a thick layer, interposed between? 

 the tegmental and basal portions of the cerebral peduncle." It begins below at 

 the superior border of the pons and extends upwards into the hypothalamus 

 The margins of this layer of dark-coloured substance come to the surface at the*' 



