78 THE CEREBRUM. 



is formed by a gray core derived from the ependymal lining of the 

 embryonic ventricle. The gray core is surrounded by a white 

 sheath of medullated fibers running longitudinally; posterior 

 to the bulb these fibers form 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 sub- 

 stance forms the terminal nucleus of the olfactory nerves and gives 

 origin to the fibers of the olfactory tract. 



Olfactory Tract (Tractus olf actor ius). The tract is triangular 

 in section, slightly more than an inch long and one line in width 

 (Fig. 26). It is partially concealed in the olfactory sulcus, and 

 is a continuation backward of the medullated mitral axones which 

 ensheath the gray core of the olfactory bulb. At its posterior end 

 the olfactory tract divides into three striae lateral, intermediate 

 and medial, two of which are easily seen. These stria olfactories 

 are continuous with the three angles of the tract. The lateral 

 and medial striae diverge and inclose the olfactory triangle between 

 them. The lateral stria (stria lateralis] courses outward and 

 backward 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, circumambiens 

 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 medialis) bends sharply 

 inward, toward the median line, and runs between the triangle 

 and parolfactory area (of Broca). Its fibers turn into Broca's 

 area and the gyrus cinguli, chiefly, but some of them enter the 

 triangle, the gyrus subcallosus, and the hippocampus through 

 septum pellucidum and fornix. Thus the medial and lateral 

 striae unite the opposite ends of the gyrus fornicatus. From the 

 dorsal angle of the olfactory tract, a bundle of fibers proceeds 

 into the triangle and frontal lobe, constituting the intermediate 

 stria (stria intermedia). The intermediate olfactory stria is often 

 not visible on the surface, as it turns at once upward into the 

 frontal lobe. Upon reaching the level of the anterior commissure, 

 it passes through that commissure to the opposite hemisphere, 

 where it divides into two bundles; one of which (the commissural 



