The pyramid tract in the Canadian porcupine (Erethizon dorsatus Linn.)

Abstract

The nerve fibers of the pyramid tract pass caudalwards from their cells of origin in the motor area of the cerebral cortex and are homo-lateral until the lower part of the medulla oblongata is reached. At this level, in the majority of mammals that have been examined, most of the fibers decussate and take up a position in the lateral column of the spinal cord on the opposite side, just ventral to the posterior horn, forming the crossed lateral pyramid tract. A few fibers remain uncrossed and are found in the lateral column on the same side, constituting the direct lateral pyramid tract.

In man and the anthropoid apes a second uncrossed tract is found in the ventral column—the direct ventral pyramid tract. In mammals lower in the scale than the anthropoids, it is generally held that this direct ventral tract is not represented at all.

This disposition of the pyramid tract fibers is not found in all mammals however. In the guinea-pig (v. Bechterew, Reveley), mouse (v. Lenhossek), rat (Flechsig, King) and squirrel (Weigner), and in the monotremes and marsupials (Kölliker and Ziehen), the crossed fibers run in the dorsal and not in the lateral column of the cord.

In the spring of the present year, I obtained some full-grown porcupines with the object of investigating the course of the pyramid tract fibers in this animal. The left motor cortex was located by electrical stimulation and then extirpated in the usual way. At the end of about a fortnight after the operation in each case the animal was killed, the brain and cord removed, stained by the Marchi method and sectioned at all levels. A full description of the resulting degenerations will be published later; in this preliminary communication only the most important points will be mentioned.