Page:Popular Science Monthly Volume 43.djvu/395

Rh layer of the spinal cord. The bundle of nerve fibers is known as the solitary tract. Although the relations are complicated and not easily rendered clear, I hope enough has been said to demonstrate that the dorsal zone always remains what it is at first—the zone into which the ganglionic fibers enter and in which they chiefly ramify.

As every one knows, the two largest divisions or parts of the human brain are the cerebrum or hemispheres and the cerebellum. These, we have now learned, are both structures developed exclusively from the dorsal zones of His, and have therefore a very different morphological value from what has hitherto been assumed—not being modifications of the whole brain, but only local developments of the dorsal half of the brain. Just as primitively the medullary fibers which arise in the dorsal zone pass into the ventral zone, so in the specialized cerebral hemispheres and in the cerebellum there arise very numerous nerve fibers, but these still obey the primal law and take their courses into the portions of the brain representing the ventral zones, and thence the fibers are distributed to their various destinations. Until the relations of the zones to the nerve fibers, on the one hand, and to the hemispheres and cerebellum on the other, had been embryologically determined, it could not be known that the course of the cerebral and cerebellar fibers is in accordance with a fundamental law of nervous organization. We can foresee, though somewhat vaguely, that essential physiological deductions will follow the application of the law to the study of the functions of the brain.

The relations of the zones in the entire brain are indicated by the diagram on page 374, which scarcely calls for comment, since it sufficiently explains itself. I need only add that the position of the dividing line of the zones in the region of the corpora quadrigemina is somewhat uncertain. In the embryo this region is known as the mid-brain, and shows the primary division very clearly; but as the further development has not been worked out properly yet, we can not decide positively as to the exact demarcation of the zones in the adult.

The ventral zone of the brain may be defined, as we have already learned, as the territory of the medullary fibers, for it furnishes the pathway for those fibers to collect in bundles, which may either form nerve roots (ventral or lateral), or may cross, as so-called commissural fibers, from one side to the other in order to establish the nervous connection between the two halves of the