INTERNAL STRUCTURE OF THE MESENCEPHALON. 493 
quadrigeminal bodies is largely composed of a mass of gray matter which, in 
transverse section, presents an oval outline (Fig. 567, p. 496). This central 
nucleus is, to a large extent, encapsulated by white matter. Numerous cells of 
different sizes are scattered throughout it, and the whole mass is pervaded by an 
intricate interlacement of fine fibres, which are derived from the lateral fillet 
and the inferior brachium. 
In transverse sections, through this region, the lateral fillet is seen to abut 
against the outer margin of the central nucleus. Many of the fibres of this tract 
enter it at once and become dispersed amongst its cells; others sweep over its 
dorsal surface, so as to give it a superficial covering; whilst a third group is carried 
in the form of a thin layer inwards on its ventral aspect, so as to mark it off from 
the subjacent Sylvian gray matter of the aqueduct (Fig. 367, p. 496). In this 
manner, therefore, the inferior quadrigeminal nucleus becomes partially cireum- 
scribed by the fibres of the lateral fillet. Several of the lateral fillet fibres, which 
proceed over the superficial or dorsal aspect of the nucleus, reach the mesial plane 
and form a loose decussation with the corresponding fibres of the opposite side. 
The intimate connexion which is thus exhibited between the fibres of the lateral fillet and 
the nucleus of the inferior quadrigeminal body is very significant. It has already been shown 
that the lateral fillet, to a large extent, comes from the ‘nuclei of termination of the cochlear 
nerve of the opposite side, although most of its fibres have to pass through several nuclear inter- 
nodes before they reach the inferior quadrigeminal body. We must “associate, therefore, the 
. inferior quadrigeminal body, and also the corpus geniculatum internum, which likewise receives 
lateral fillet fibres, with the organ of hearing. 
This view of the inferior quadrigeminal bodies is supported both by experimental and by 
morphological evidence. Speaking broadly, it may be stated that the ‘inferior quadrigeminal 
bodies are only present as distinct eminences in mammals, and then they are invariably correlated 
with a spirally-wound and well-developed cochlea. That they have nothing to do with sight 
is shown by the fact that, when the eyeballs are extirpated in a young animal, the inferior 
quadrigeminal bodies remain unaffected, whilst the superior quadrigeminal bodies after a time 
atrophy (Gudden). When, on the other hand, the cochlear terminal nuclei are destroyed, fibres 
which have undergone atrophy may be followed to the inferior quadrigeminal bodies of both 
sides, but particularly to that of the opposite side (Baginski, Bumm, and Ferrier and Turner). A 
very considerable tract of ascending fibres takes origin within the inferior quadrigeminal body 
and passes upwards, in the inferior “brachium, into the tegmentum subjacent to the ternal geni- 
culate body. Within the tegmentum they proceed up to the optic thalamus (Ferrier ~ and 
Turner). 
Superior Quadrigeminal Bodies (colliculi superiores)—The superior quadri- 
geminal body presents a more complicated structure (Fig. 566). Supertficially, it is 
coated with a layer of white matter, which is termed the stratum zonale. Underneath 
this there is a gray nucleus, called the stratum cinereum, which in transverse section 
exhibits a crescentic outline and rests in a cap-like manner upon the subjacent part 
of the eminence. The succeeding two strata, which respectively receive the names 
of stratum opticum and the stratum lemnisci, present this feature in common that 
they are composed of gray matter, traversed by numerous fibres. The source from 
which the fibres are derived differs, however, in each case. 
Nerve-fibres reach the superior quadrigeminal body through—(1) the lateral 
and mesial fillets, and (2) through the superior brachium. The fillet fibres enter 
the stratum lemnisci, and, in all probability, end there. The superior brachium 
contains fibres of two different kinds, viz. fibres from the optic tract and fibres from 
the cortex of the occipital lobe of the cerebrum. By the former it is connected 
with the retina, and by the latter with the visual centre in the occipital region of 
the cerebral cortex. The retinal fibres, for the most part, spread out on the surface 
of the quadrigeminal body and form the stratum zonale. Most of them dip down 
into the substance of the body and end in connexion with the cells of the deeper 
layers ; several, however, are carried across the mesial plane, to end in the superior 
quadrigeminal body of the opposite side. The occipital fibres, and probably also 
some of the retinal fibres, enter the stratum opticum. The fibres from the occipital 
cortex form part of the optic radiation, and the course which they pursue will be 
dealt with later on. 
Tegmental Portions of the Crura Cerebri (tegmenta).—The tegmentum of 
the crus cerebri may be regarded as the continuation upwards of the formatio 
