Rubro-spinal fasciculus (crossed descending tract of the red nucleus).—Formed by axones of the red nucleus, it im mediately crosses through the ventral tegmental decussation (foreli) and proceeds lateralward to the angle between the medial and the lateral fillet (Fig. 59). In the lower part of the mid-brain it is imbedded in the medial part of the lateral fillet. The rubro-spinal tract runs through the medulla and descends in the spinal cord to the lumbar region; it ends in the gray crescent and in motor nuclei of the brain-stem.
Thalamo-spinal Fasciculus.—The rubro-spinal tract, from the lower end of the mid-brain downward, is associated with a larger bundle originating in the thalamus, called the thalamo spinal fasciculus, and with a smaller tract from the quadri geminal colliculi already described as the lateral tecto-spinal fasciculus; also with the lateral reticulo-spinal fasciculus. All these fasciculi terminate in relation with motor nuclei, cranial and spinal (Figs. 61 and 63).
Thalamo-olivary Fasciculus.—The thalamo-olivary bundle is a loose strand of fibers traversing the reticular formation lateral to the medial longitudinal bundle, in the upper part of the mid brain; in the lower region of the mid-brain it runs closer to the median line and is dorsal to the fibers of the brachium conjuncti vum as they are about to enter the decussation. The thalamo olivary bundle probably rises in the thalamus and ends in the olivary nucleus of the medulla. It becomes a distinct visible tract at the upper end of the pons (Figs. 61 and 63).
The gustatory fasciculus has been traced by Otto May and Sir Victor Horsley (Brain, 191o). The taste tract runs up through the mid-brain dorso-lateral to the medial longitudinal bundle and chiefly dorsal to, but partly intermingled with, the thalamo-olivary fasciculus. It rises in the nucleus of the soli tary tract (nucleus of Nageotte) near the junction of the pons and medulla; its termination is said to be in "the inner part of the dorsal third of the nucleus lateralis" of the thalamus. It was first observed by Probst (1899) and Lewandowsky (1904).
The mesencephalic root of the trigeminal nerve (Fig. 6o) rises in the lateral part of the central gray matter in the mid brain. It is a motor root. It occupies a thin crescentic area just at the lateral border of the stratum griseum centrale which thickens as it proceeds downward toward the pons. This root
extends the entire length of the mid-brain; but it is made up of very few fibers in the upper part and only assumes a distinct shape and outline when the level of the inferior quadrigeminal colliculus is reached. It is continued to the middle of the pons in the same lateral relation to the gray substance; and medial to the brachium conjunctivum cerebelli it joins the main part of the motor root and bends forward toward the anterior sur face. According to Otto May and Horsley the mesencephalic root extends only to the semilunar ganglion. It is intermingled in the mid-brain with ascending fibers of the sensory root of the trigeminal nerve; these fibers terminate about the cells of the mesencephalic nucleus and are probably reflex in function. J. B. Johnston's view of the mesencephalic nucleus is very different. He believes it represents an enveloped ganglion of pear-shaped cells; that its fibers correspond to the peripheral processes of ganglion cells elsewhere; and that they are afferent in function and join the sensory root of the trigeminal nerve.
The Quadrigeminal Lamina (Lamina quadrigemina).—The quadrigeminal lamina forms the fourth great division of the mid-brain. It rests upon the dorsum of the tegmenta, entering into a large part of the posterior surface of the mesencephalon. It is the tectum. A crucial groove shapes its surface into four eminences, called colliculi (colliculi superiores and inferiores) (Fig. 56).
The colliculus superior of either side is larger than the infe rior colliculus and is circular in outline. It has resting upon its medial half the pineal body. It is joined to the lateral genicu late body by a band of fibers almost entirely concealed by the pulvinar of the thalamus. That band is the brachium superius. The superior colliculus is made up of gray substance for the most part (Figs. 58 and 61). It is composed of a superficial white layer, the stratum zonale, and a thick laminated gray layer, the stratum griseum. Within the stratum griseum many fibers end; a few from the lateral fillet, spino-tectal fasciculus, all of the superior fillet, and nearly all of the brachium superius. The stratum griseum gives origin to the tecto-spinal fasciculi and probably to a few fibers that run through the brachium superius into the optic nerve. It constitutes an optic-reflex center.