Commissural Fibres of the Cerebro-Spinal Axis

Along the whole of the spinal cord and brain, fibres are crossing from one side to the other. Many of these are the decussating projection fibres, but others are simple commissural fibres. Thus there are two transverse commissures, an anterior white, and a posterior grey, extending the complete length of the spinal cord and containing fibres of both systems.

In the cerebellum many simple commissural fibres run from one hemisphere to the other through the vermis. It is in the cerebral hemispheres however that large commissural bands without projection fibres, can be most clearly differentiated. As already seen the largest of these is the corpus callosum - others of the same nature are the anterior commissure, and, to a lesser extent, the fornix. The fibres of the corpus callosum connect the entire cortex of one hemisphere with that of the other (Fig. 72).

The anterior commissure joins the temporal lobes of opposite sides. The transverse fibres of the fornices run from one hippocampal convolution to the other.

Three of the sensory cerebral nerves, viz. olfactory (i), optic (ii), and auditory (viii); present so much variation from the ordinary arrangement that a special description of them is necessary.