THE NERVOUS SYSTEM. 
INTERNAL STRUCTURE OF THE MESENCEPHALON. 
When transverse sections are made through the mesencephalon the aqueduct of 
Sylvius is seen to be surrounded by a thick layer of gray matter, which receives the 
name of the Sylvian gray matter or the central gray matter of the aqueduct (stratum 
griseum centrale), On the dorsal aspect of the Sylvian gray matter the corpora 
quadrigemina form a layer which separates it from the surface, and to which the 
SYLVIAN 
CORPORA QUADRIGEMINA 
——— GREY MATTER 
AQUEDUCT OF SYLV‘US., 
POSTERIOR 
term lamina quadrigemina is applied. 
On the ventral and lateral aspects 
of the Sylvian gray matter are the 
tegmental portions of the crura 
LONGITUDINAL 
BUNDLE 
cerebri, whilst, itervening between 
each of the latter and the correspond- 
ing crusta, there is a conspicuous mass 
of dark pigmented matter termed 
the substantia nigra. 
Sylvian aqueduct and Sylvian 
\ gray matter (aqueductus cerebri— 
stratum griseum  centrale).— The 
aqueduct of Sylvius is the canal 
which leads from the fourth ven- 
tricle below, upwards through the 
mesencephalon, to the third ventricle 
above. It is not quite three-quarters of an inch in length, and it les 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 individuals. 
The aqueduct of Sylvius is lined by ciliated epithelium, and outside this is the 
thick layer of Sylvian 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 Sylvian gray matter are numerous nerve-cells of varying forms and 
S1Zes, ‘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 oculo-motor 
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 
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, 
the brown-coloured cells stand out very conspicuously, even under low powers. 
The substantia nigra is, in reality, disposed in the form of a thick band, interposed 
between the tegmental and pedal portions of the crus cerebri. It begins below at 
the upper border of the pons Varolii and extends upwards into the subthalamic 
region. The margins of this band of dark-coloured substance come to the surface 
at the oculo-motor and the lateral sulci of the mesencephalon, and its inner part is 
traversed by the emerging fascicles of the oculo-motor nerve. It is not equally 
thick throughout. Towards the lateral sulcus it becomes thin, whilst it thickens 
considerably near the inner aspect of the crus cerebri. The surface of the sub- 
stantia nigra, which is turned towards the tegmentum, is concave and uniform ; the 
opposite surface is convex and rendered irregular by the presence of numerous 
slender prolongations of the substance into the crusta. 
Abie morphological and physiological significance of the substantia nigra 1s not 
fully understood, and the connexions established by its cells are imperfectly 
known. 
Inferior Quadrigeminal Bodies (colliculi inferiores)—Kach of the inferior 
SULCUS LATERALIS 
OCULOMOTOR 
NERVE 
SULCUS OCULOMOTORIUS 
364.—DIAGRAMMATIC VIEW OF THE CuT SURFACE OF 
A TRANSVERSE SECTION THROUGH THE UPPER PaRT 
OF THE MESENCEPHALON,. 
Fia. 
