Myeloarchitectonics of the Posterior Commissure of the Dog's Brain.

The purpose of this study was to trace the course of the posterior commissure and to mark out its parts. To carry out this task, 5 series of sections of the dog's brain were made. These were: 3 series of sections in the frontal, sagittal, and horizontal planes, stained by the Weigert-Wolters method, and, 2 series of sections stained, using alternately Kliiver's and Nissl's methods. Topography of the Posterior Commissure The posterior commissure of the dog lies on the border of the dorsal portion of the diencephalon and the mesencephalon. It belongs to a complex of commissures, which seemingly form a whole. This complex includes the habenular commissure, posterior commissure, and commissure of the anterior quadrigeminal bodies. It forms a twisted band of varying width in the diencephalon and mesencephalon. In the sagittal section, the band rather resembles the mark of interrogation in shape (Fig. 1). The proper posterior commissure extends from the base of the habe-nulae to the anterior prominences of the quadrigeminal bodies. Here, it terminates merging with the commissure of the anterior quadrige-minal bodies without any distinct boundary. Out of the 3 above-mentioned commissures, the posterior commissure has the largest number of fibres. It is situated in the posterior wall of the third ventricle, just at the entry of the cerebral aqueduct. Above the posterior commissure there is the pineal body, while dorsally and anteriorly it verges on the habenular commissure and the habenulae separated from the commissure by the pineal recess. The posterior commissure is built in the central grey matter, which joins the grey matter of the wall of the third ventricle orally and passes into the grey matter of the bottom of the fourth ventricle inferiorly. The cells of the central grey matter in this region of the brain are grouped in nuclei, of which the most important ones are the nucleus commissu-rae posterioris, nucleus interstitialis, nucleus of Darkschewitsch, and the nuclei nervi oculomotorii. The nucleus commissurae posterioris, regarded by most anatomists as common to the fibres of the posterior commissure and those of the medial longitudinal fascicle (Ramony Cajal 1904, K a p p e r s 1947, Kuhlenbeck 1929), consists of two parts: medial and lateral (Fig. 2). It is situated ventrally to the pretectal and lenticular nuclei and dorsally to the nuclei mentioned above. It extends orocaudally over a distance of about 1150 and receives the fibres of the optic tract …