LOBUS OLFACTORIUS ANTERIOR.
The bulbus olfactorius presents usually an oval form—an ellipse or a vertically flattened band—and constitutes an enlargement of the tractus olfactorius in front. On the under surface, delicate threads, the fila olfactoria, pass out and descend into the nasal fossa through the apertures of the lamina cribrosa. They are disposed in two series and may be designated as the fila olfactoria medialia and lateralia. They are so delicate that they are always broken off when the brain is removed.
The tractus olfactorius lies as a white strand in the olfactory sulcus and, on cross-section, reveals the form of a triangle with base below and apex above buried in the sulcus. The hind part of the tract, toward the tuberculum olfactorium, is narrow and somewhat compressed.
The tuberculum olfactorium, into which the tractus is prolonged behind, appears in its true form only after the bulb and tract have been raised from the olfactory sulcus and the latter itself rendered more gaping by pulling apart the hounding convolutions. Then the tuberculum appears as a pyramidal elevation, whose apex pene trates the sulcus and whose base forms an irregular triangular field, the trigonum olfactorium.
From the tuberculum proceed the medial and lateral olfactory convolutions whose courses are as follows : The gyrus olfactorius medialis runs as a narrow convolution medially. In front, it is bounded by the medial posterior branch of the sulcus olfactorius; medially and behind, by the sulcus parolfactorius posterior (the fissura prima of His). A white fascic ulus of fibres, the stria offactoria medialis, the continuation of the medial strand of the olfactory tract, streams into the gyrus olfactorius medialis, soon to become lost in the • gray substance of the convolution.
On following the medial gyms farther, it is found to radiate within a small field on the medial surface of the hemisphere, that lies immediately below the rostrum of the corpus callosum and is bounded both in front and behind by a small fissure. The furrow in front is the sulcus parolfactorius anterior, while the one behind is the continuation of the sulcus parolfactorius posterior, already mentioned. The small field is called the area fiaroljactoria, or field of Broca. It joins the gyrus olfactorius medialis with the gyms fornicatus, particularly with the gyms cinguli (Figs. 18 and 23), and thus establishes the connection of the lobus olfactorius anterior with the central region of the rhinencephalon. The gyrus olfactorius lateralis runs laterally. In the foetal brain of from four to five months, one readily recognizes that the gyrus passes outward towards the Sylvian fossa, its so-called front limb proceeding from the trigonum almost at right angles ; then, at the medial margin of the fossa and after an acute bend, the hind limb runs backward and medially to the anterior border of the gyrus hippocampi. Here the gyms ends, to a certain extent, in two claws, the medial one of which is known as the gyrus semilunaris rhinencephali, and the lateral as the gyrus ambiens rhinencephali. The
fissure separating the claws is the sulcus semiannularis (Figs. 25 and 26). In conse quence of the subsequent strong development of the frontal and temporal lobes, which approach each other more and more, the angle formed by the two limbs of the gyrus torius lateralis becomes progressively more acute, although the demarcation of the gyrus from the insula is still distinct. In the later stages, the limbs become more closely approximated and the apical part of the convolution is incised by the sulcus centralis insulae, which meanwhile has been formed. The result is, that the previous connection of the two limbs, as well as the marcation of the convolution towards the insula, vanishes, the convolution now appearing to expand within the substance of the insula.
Since these relations persist in the adult, it was assumed that the lateral olfactory convolution, which bounds the insula medially, belonged to the island ; hence it was designated the limen insulae. The latter, however, belongs to the rhinencephalon and represents the gyrus olfactorius lateralis, which is subdivided into a front and hind limb—pars anterior and posterior. The area enclosed by the limbs was named by Retzius, the angelus gyri olfactorii lateralis.
The pars anterior usually appears as a fairly broad convolution, which extends from the tuberculum olfactorium outward and somewhat obliquely backward and is sep arated from the substantia perforata anterior by a fissure, the sulcus arcuatus cephali, that follows the gyrus olfactorius lateralis medially as far as the gyrus hippocampi.
Laterally and in front, the pars anterior joins the orbital convolution to form the gyrus of Retzius, which medially is bounded by the postero-lateral branch of the sulcus olfactorius. The gyrus is commonly simple, but may be divided into two parts by a short fissure ; likewise, it may be subdivided by a longitudinal fissure into two subsidiary convolutions, an anterior and a posterior.
The stria olfactoria lateralis passes as a white fibre-bundle outward along the pars anterior toward the angulus gyri olfactorii lateralis, here lying quite near the substantia perforata anterior. It then bends backward in the angle and later disappears. Occa sionally the lateral olfactory root consists of two bundles, of which the medial follows the border of the substantia perforata, until it is finally lost within the substance. It is to be further noted, that a third or middle root may be found between the lateral and medial ones. It soon vanishes, however, within the substantia perforata.
After recurving in the angle, the lateral olfactory convolution, as the hind limb or pars posterior, continues inward and backward toward the front end of the gyrus ' hippocampi.
On examining more closely the antero-median surface of the gyrus hippocampi in the adult brain, one sees the convolutions in which the posterior limb fades away— the gyrus semilunaris medially and the gyrus ambiens laterally. The gyrus ambiens arches around the gyrus semilunaris and then is lost within the uncus.