152 THE CEREBRUM. 



The Tegmenta (Figs. 46, 47 and 48). The posterior divisions 

 of the pedunculi cerebri, which cover the other divisions, are in 

 consequence called the tegmenta (tegmentum a cover). They 

 are united by a median raphe and fit ventrally into the concavity 

 of the substantia nigra. They are bounded by the lateral sulcus 

 of the mid-brain on the free side, where each tegmentum presents 

 a triangle, bounded by the sulcus lateralis, in front; by the bra- 

 chium inferius, above; and, inferiorly, by the lateral fillet. Dor- 

 sally, the tegmenta fuse with the quadrigeminal lamina except 

 along the median line, where the cerebral aqueduct separates them. 

 Each tegmentum at the superior end blends with the thalamus, 

 and helps to form the tegmental hypothalamic structures. Im- 

 bedded in that superior portion is the red nucleus (n. ruber) of 

 the tegmentum. (See tegmental hypothalamic region.) Inferiorly, 

 the tegmenta are continued into the reticular formation of the 

 pons. 



The Cerebral Aqueduct (Aqueductus Cerebri, Sylvii, Figs. 17, 27 

 and 47). The aqueduct is a very slender canal connecting the 

 third and fourth ventricles. So it is the "her a tertia ad quar- 

 tum ventriculum." It is situated in the median line under the 

 quadrigeminal lamina and produces a slight beveling of the dorso- 

 medial borders of the tegmenta. It is a half-inch long. In shape 

 it is v-like, above; elliptical, in the middle, with a vertical major 

 axis; and T-form, below, where it joins the fourth ventricle. 

 Its height varies between a sixteenth and an eighth of an inch. 

 Like other ventricles, it is lined with ependyma. A layer of gray 

 matter, thickest on the sides and floor, surrounds the aqueduct 

 of the cerebrum. This is the stratum griseum centrale, which 

 gives rise to some of the fibers of the posterior commissure. The 

 stratum griseum centrale is continuous with the gray matter of 

 the fourth ventricle. In the ventral part of it are the nuclei of 

 the oculomotor (third), the trochlear (fourth) and the trigeminal 

 (fifth) cerebral nerves. 



Nuclei of the Oculomotor and Trochlear Nerves (Figs. 47 and 

 48). Both nuclei together extend the entire length of the aque- 

 duct, and the oculomotor is prolonged into the wall of the third 

 ventricle, where it receives a bundle of fibers from the opposite 



