THE MID-BRAIN 899 



fusiform or angular in outline. The cells are characterized by a pigment (marked 

 only in man) which varies from a pale brown in the young to an absolute blackness 

 in the very aged. The axones arising frorr the cells proceed in various directions 

 toward the tegmentum and crusta, but their exact course is not known. Experi- 

 mental excitation of this ganglionic mass elicits movements of deglutition accom- 

 panied by respiratory changes. Mellus has found in the monkey that a portion 

 of the pyramidal tracts is interrupted in the substantia nigra. 



The corpora quadrigemina are largely composed of gray substance, but the 

 superior and inferior corpora quadrigemina differ distinctly in structure. 



The superior corpora quadrigemina or pregemina present a true cortical type, 

 which is more evident in the optic lobes of lower vertebrates. In man the thin, 

 outermost white layer the stratum zonale is an expansion of the optic tract. 

 Beneath this lies a gray nucleus, with numerous small cells the stratum cinereum 

 a cup-like layer of crescentic outline on trans-section. The succeeding ental 

 layer is a white stratum, also derived from the optic tract the stratum opticum. 

 Between this and the underlying stratum lemnisci is a second gray layer, less - 

 defined because of the diffuse interlacing of white fibres. " 



Each superior quadrigeminal body is one of a series of primary centres of vision 

 related more to eye-muscle reflexes resulting from optic and auditory stimuli 

 than to actual light and color perception. Fibres from the retina, for the most 

 part, form the stratum zonale and end in the ganglionic gray; others enter into the 

 formation of the stratum opticum. Return fibres from the occipital cortex also 

 enter the stratum opticum. The retinal and occipital fibres determine the forma- 

 tion of the superior brachium. Other fibres reach the superior quadrigeminal 

 body through the lateral and medial lemnisci from both sides to end in rela- 

 tion with the deeper cells of the stratum cinereum. The connections of the 

 superior quadrigeminal body with the cochlear centres afforded by the lateral 

 lemniscus establishes the so-called optic-acoustic reflex path. 



The inferior corpora quadrigemina or postgemina are more homogeneous in 

 texture, comprising a pair of compact ganglia which on trans-section have the 

 shape of biconvex lenses, encapsulated by white substance. The cells are small, 

 multipolar, and very numerous, embedded in a fine molecular groundwork. 

 The white stratum zonale is principally derived from the fibres of the lateral 

 lemniscus, which terminate in the central gray of the inferior quadrigeminal body 

 as well as in the internal geniculate body. The axones of the cells in the inferior 

 quadrigeminal body course cephalad in the inferior brachium, dip beneath the 

 internal geniculate body into the tegmentum, and proceed to the thalamus. The 

 inferior quadrigeminal bodies are important links in the chain of the auditory 

 neurone system, and are special localities for the reflexion of auditory impulses. 



The tegmentum of the mid-brain is continuous with the like formations in the . 

 hind-brain stem and consists of longitudinal fibre bundles intersected by transverse 

 arched fibre systems with gray substance irregularly scattered in the interstices, 

 composing the formatio reticularis. In its ventral portion, on either side, and at 

 the level of the superior quadrigeminal body, lies a gray ganglionic mass, the 

 red nucleus. 



The red nucleus or rubrum (nucleus tegmenti; nucleus ruber), so termed from its 

 reddish tinge in the fresh brain, which it owes to the pigmentation of its cells as 

 well as to its great vascularity, is found subjacent to the superior quadrigeminal 

 body in those section-levels where the substantia nigra has its greatest expansion. 

 In trans-sections its outline is irregularly circular; in sagittal sections an elongated 

 oval. The red nucleus is the end station for the majority of the decussated 

 fibres of the superior peduncles of the cerebellum, for fibres from the cerebral 

 cortex, and from the corpus striatum. These fibre bundles form for the nucleus 

 a capsule which is thicker on its ectal surface. From the cells of each of the 



