SURFACES. 163 



The descending root of the trigeminal nerve (Fig. 48) 

 rises in the ventro-lateral part of the central gray matter in the 

 mid-brain and is, therefore, called the mesencephalic root. 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 rela- 

 tion to the gray substance; and medial to the brachium conjunc- 

 tivum cerebelli it joins the main part of the motor root and bends 

 forward toward the anterior surface. 



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. A 

 crucial groove shapes its surface into four eminences, called 

 colliculi (colliculi superiores and inferiores) (Fig. 44). 



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 geniculate 

 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. 46 and 47). 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, all of the superior fillet, and nearly all of the 

 brachium superius. The stratum griseum gives origin to the 

 anterior longitudinal bundle and, probably, to a few fibers that 

 run through the brachium superius into the optic nerve. It 

 constitutes an optic-reflex center. 



The colliculus inferior of the corpora quadrigemina is elong- 

 ated transversely (Fig. 44). It is joined to the medial geniculate 

 body by an oblique ridge, called the brachium inferius, and it 

 forms the termination of two ridges that approach it from below, 



