The exploration and surgical treatment of the lesions of the 3rd ventricle may be carried out without sectioning the anterior columns of the trigonum. The body of the trigonum and the choroid tissue are lifted sometimes at the cost only of the sectioning of the thalamostriated vein. The absence of mnesic trouble particularly in benign tumours of the third ventricle is the essential advantage of same. Access is very much wider particularly for the posterior part of third ventricle.