CEREBRUM SURFACES The mid-brain has four surfaces, viz., the anterior and pos terior, which are free, and the superior and inferior, representing the ends of the cylinder, which are attached. The two latter are nearly parallel with each other and are formed by section.
The superior surface, sloping downward and forward, meets the anterior surface at an acute angle. Its inclination is that of the posterior end of the floor of the third ventricle. Lateral to the floor of the ventricle it is attached to the thalami and in ternal capsules. The blending of it with the thalami forms the structures of the tegmental hypothalamic regions and the con tinuations of the extreme lateral portions, the bases pedunculi, enter into the internal capsules of the hemispheres. In the median line behind the third ventricle it is attached to the posterior commissure. The superior surface is 3.7 cm. (1.5 in.) broad.
The inferior surface joins the upper surface of the pons. It is a little narrower than the superior surface. It is about 3 cm. (1.25 in.) broad and measures 2.5 cm. (r in.) dorso ventrally.
The anterior surface (ventral) of the mid-brain looks forward and downward (Figs. 57, 59 and 6o). It is deeply grooved longitudinally by a median sulcus, the fossa interpeduncularis, and is slightly concave from above downward. It is separated on either side from the posterior surface by the sulcus lateralis mesencephali. Though partially concealed by the temporal lobes of the cerebrum, the anterior surface is unattached. It is formed by a prominent band, the basis pedunculi at either side; and by a median structure, the posterior perforated substance, which is inclosed between the two bases. The posterior per forated substance forms the floor of the median sulcus. The inner border of the basis pedunculi is free and overhangs the perforated substance slightly. Thus is formed the oculomotor groove (sulcus nervi oculomotorii) between the basis and per forated substance whence the third cerebral nerve emerges from the brain. The fourth nerve courses forward over the anterior surface, but is not attached to it.
The posterior surface (dorsal) of the mid-brain (Fig. 56), though free, is entirely concealed by the cerebellar and cerebral hemispheres. It forms part of the floor of the transverse fissure of the cerebrum and is covered by pia mater. The lateral sulcus bounds it on each side. From the sulcus lateralis it elevates abruptly toward the median line, where it presents a longi tudinal groove. This produces two ridges which are sub divided by a transverse groove into the four eminences, the colliculi of the corpora quadrigemina. On either side, anterior and a little lateral to the quadrigeminal bodies, is the medial geniculate body, joined to the inferior quadrigeminal colliculus by an oblique ridge, called the brachium inferius. The nearly parallel longitudinal ridges below the corpora quadrigemina are formed by the brachia conjunctiva of the cerebellum. The bottom of the groove between them is formed by the superior medullary velum (of Vieussens), whence the trochlear nerve (fourth) is seen issuing.
I. Corpora quadrigemina and brachia.
Mid-brain 1. Tegmenta II. Pedunculi 2. Substantia nigra 3. Bases pedunculi.
The four colliculi of the corpora quadrigemina and the four brachia connecting them with the geniculate bodies constitute the quadrigeminal lamina, which forms the greater part of the posterior surface of the mid-brain (Fig. 56). It is also called the tectum. This lamina rests upon the dorsum of the pedun culi cerebri. The lamina quadrigemina presents a small median triangle between the superior colliculi and the habenula, called the subpineal triangle, in which the pineal body rests. The lamina is invested with pia mater.
The pedunculi cerebri are made up of three great divisions, as shown above; named from before backward, they are: (1) The bases pedunculi, comprising the anterior part; (2) the substantia nigra, which is the middle part; and (3) the tegmenta, which are united by a median raphe and lie in the posterior region next the quadrigeminal lamina. In the median plane between the quadrigeminal lamina and the tegmenta runs the cerebral aqueduct.