GYRI ANDREAE RETZII.
These, also known as the callosal convolutions, represent rudimentary gyri, which appear as round or oval elevations on the medial surface of the gyrus- hippocampi, beneath the splenium and in the angle formed by the dentate and hippocampal gyri. They are not constant and may be little more than mere suggestions, or, when strongly developed, may resemble a spirally wound cord. Zuckerkandl desig nates them as callosal convolutions, and Giacomini reckons them, in view of their structure, as belong ing to the hippocampus. G. Retzius named the convolutions in honor of their discoverer, Anders Retzius, his father, the gyri Andreae Retzii (Fig. 34).
Summary. Taking a general survey of the entire rhinencephalon (Fig. 36), we distinguish a periph eral and a central region.
The peripheral region includes a front and a hind part, the lobus olfactorius anterior and the lobus olfactorius posterior. The central region embraces a large annular tract on the medial surface of the hemisphere, and includes the gyrus fornicatus and the gyrus dentatus.
Peripheral and central regions are closely united with each other, the lobus olfactorius anterior being connected with the gyrus fornicatus and the lobus olfactorius posterior with the gyrus dentatus. Moreover, the lobus olfactorius anterior is
connected, on the one hand, with the gyrus cinguli by means of the gyrus olfactorius medialis and, further along, the area parolfactoria ; on the other hand, it is joined with the front end of the gyrus hippocampi by means of the gyrus olfactorius lateralis. The lobus olfactorius posterior is connected with the gyrus dentatus by means of Broca's diagonal band, the gyrus subcallosus and the induseum covering the corpus callosum. As will appear later, the olfactory centre is supposed to lie chiefly in the cortex of the gyrus hippocampi. Therefore, the impulses transmitted from the nasal mucous membrane by the fila olfactoria must be carried from the bulbus olfactorius and transferred to the central region of the rhinencephalon. The course of this olfactory tract will best explain the connections of the individual parts of the rhinencephalon (page 144).