Direct Vestibulo-Spinal Fibers in the Medial Longitudinal Fasciculus of the Cat.

In spite of a considerable amount of experimental work on the fiber connections of the vestibular nuclei, there are still some points on which there is disagreement. The marked disturbance in posture and eye movements resulting from vestibular lesions makes it highly desirable to have a very accurate knowledge of the nervous circuits arising from the various vestibular nuclei. To contribute further data on several of the disputed questions, we have produced mechanical lesions in various parts of these nuclei in 20 cats. The cat was used not only because the vestibular system is well developed in this animal, but also because in the course of various other degenerations experiments carried out in this laboratory on the cat, the vestibular nuclei have been unintentionally damaged, thus incidentally contributing data on the vestibular system. After allowing 2 weeks for degeneration, the brains were kept 3 weeks in 3% potassium dichromate, changing the fluid daily during the first week. Blocks 2 to 3 mm. thick were then kept for 10 days in a relatively large quantity of 3% potassium dichromate containing not over 1/3% osmic acid. Most of the disturbing artifacts are eliminated by thus decreasing the duration in the osmic acid solution and reducing the osmic acid concentration. In most particulars our series confirm the specific conclusions of Gray: that the superior vestibular nucleus gives origin to fibers which ascend in the medial longitudinal fasciculus of the same side, that the medial (triangular) nucleus sends both ascending and descending fibers into the medial longitudinal fasciculus of the opposite side, and, of course, that the lateral (Deiters') nucleus is the origin of the well known descending lateral vestibulo-spinal tract.