Passing above the Pons Varolii, we come to an isthmus, composed of two thick strands of nerve substance connecting the pons Varolii with the cerebral hemispheres. These are called the crura cerebri. They diverge slightly in their upward course toward the hemispheres, and lie just below the corpora quadri-gemina, already referred to. Minute examination of these crura brings to light an anatomical difference which corresponds with a physiological separation between the paths taken by the sensory and motor impulses in each crus. The lower and more anterior part, which can be seen on the base of the brain, is called the base or crusta. This is made up of efferent nerve channels only. The posterior or upper part, which lies next to and is connected with the corpora quadrigemina, is called the tegmentum, and is composed of afferent fibres. Anatomically, the separation between the two is indicated by some scattered nerve cells {locus niger). The base, or crusta, which is the great bond of union between the spinal cord and the cerebral motor centres, passes into the corpus striatum; and the tegmentum, or great sensory tract, is directly connected with the optic thalamus.