The insula is continuous with the anterior perforated sub stance, and the area of transition from one to the other is called the threshold, or linen insula (Fig. 31).
Rhinencephalon.—The smelling brain belongs to the basal surface. It is retrogressive in man, being relatively larger and better developed at the fifth month in utero than in the brain of an adult. Many connected parts make it up. It is divided into two embryonic parts by the sulcus parolfactorius posterior. These are designated as the pars anterior rhinencephali and the pars posterior rhinencephali. The pars anterior of the rhinen cephalon, the olfactory lobe, embraces the olfactory bulb, tract, triangle, the medial and intermediate stria and the area parol factoria. In the pars posterior rhinencephali are included the anterior perforated substance, the gyrus subcallosus, gyrus diagonalis, the lateral olfactory stria, the limen insulm, the uncus and hippocampal formation.
Olfactory Lobe (Lobus olfactorius).—There is one lobe that is studied only on the basal surface of the fore-brain. That is the olfactory lobe (Fig. 31). Being the pars anterior rhinencephali, it comprises six connected parts; and the reason for calling them the olfactory lobe is found in the lower animals and in the human embryo, where it exists as a prominent hollow process of the cerebral hemisphere (Figs. 17 and 18).
In the horse, ox, sheep, dog, pig, etc., the olfactory lobe con tains a ventricle continuous through the intermediate stria with the lateral ventricle.
Bulbus olfactorious Tractus olfactorius Olfactory Lobe Trigonum olfactorium Stria medialis intermedia Area parolfactoria. The olfactory bulb (bulbul olfactorius) is an ovoid mass of brain matter about 12 mm. (o.5 in.) long, 4 mm. (0.17 in.) wide and 6 mm. (0.23 in.) in vertical diameter (Fig. 31). It is lodged in the olfactory sulcus of the frontal lobe and rests upon the cribriform plate of the ethmoid bone through which it receives the twenty or thirty olfactory nerves. The center of the bulb is formed by a gelatinous core derived from the ependy mal lining of the embryonic ventricle. The gray core is sur rounded by a white sheath of medullated fibers running longi tudinally; posterior to the bulb these fibers enter the olfactory tract. Five layers of gray substance thicker on the ventral side, surround the white sheath and constitute the surface of the bulb. The gray substance forms the terminal nucleus of the olfactory nerves and gives origin to the fibers of the olfactory tract.
Olfactory Tract (Tractus olfactorius).—The tract is triangular in section, slightly more than 2 cm. long and 2.5 mm. in width (Fig. 26). It is partially concealed in the olfactory sulcus and is composed chiefly of the medullated axones of the mitral and brush cells in the bulb, they form its broad basal portion; but its narrow dorsal border is made up largely of gray substance, called the cortex of the olfactory tract, and its center is formed by a gelatinous core derived from the ependyma of the embryonic ventricle. The origin of the olfactory lobe, as a hollow divertic
ulum of the telencephalon, explains this formation of bulb and tract. At its posterior end the olfactory tract divides into three stri—lateral, intermediate and medial, two of which are easily seen. These stria olfactorice are continuous with the three angles of the tract. The lateral and medial stria diverge and inclose the olfactory triangle between them.
The fibers of the olfactory tract are not continued through the striae as they appear to be; they terminate in the cortex of the tract, the olfactory triangle, the anterior perforated sub stance and the septum pellucidum, whence new sets of fibers take their origin. The lateral stria (stria olfactoria lateralis) rising from the olfactory triangle, courses outward and back ward and terminates in the uncus at the anterior extremity of the hippocampal gyrus. According to Retzius, the lateral olfactory stria terminates in the rudimentary gyri, circum ambiens and semilunaris, which form the anterior end of the hippocampal gyrus. The lateral stria bounds on the outer side the anterior perforated space. The medial stria (stria medians) bends sharply inward, toward the median line, and runs between the triangle and parolfactory area (of Broca). Its fibers rise in the olfactory triangle; they ascend through the gyrus subcallosus to the corpus callosum, around which they describe almost a complete circuit through the gyrus supracallosus (medial longitudinal stria), fasciola cinerea, and dentate fascia to the hippocampal formation. The intermediate stria (stria olfactoria intermedia) which is usually buried in the triangle and per forated substance, comprises five small strands of fibers: (r) The olfacto-hippocampal bundle of the fornix. It rises in the olfactory triangle, the anterior perforated substance and the septum pellucidum, and ends in the hippocampus. (2) The olfacto-amygdalate bundle has the same origin; it crosses almost completely through the anterior commissure and runs as stria terminalis to the nucleus amygdalae; a few of its fibers end in the thalamus on the side of its origin. (3) The olfacto-habenular bundle rises in the anterior perforated substance and septum pellucidum; it ascends to the thalamus and runs through the stria thalami to the nucleus habenulm, chiefly the opposite one. (4) The olfacto-mesencephalic bundle (basal bundle of Wallen berg) rises in the cortex of the olfactory tract; descending as far as the spinal cord, it gives off fibers to tuber cinereum, cor pus mammillare, tegmentum of mid-brain, pons, etc. (5) The commissural olfactory bundle, which extends from the cor tex of the olfactory tract, through the anterior commissure and through the olfactory tract of the opposite side to the granular and glomerular layers of the olfactory bulb.