THE MAMMALIAN NERVOUS SYSTEM 101 



olfactoria and the fibers of the second order terminating in them 

 are called tractus olfactorius (lateralis, medialis, and inter- 

 medins). The tracts of the third order are usually named by 

 hyphenated compound words, of which the second member 

 designates the center into which the tract discharges, thus 

 we have the tractus olfacto-habenularis, tractus olfacto-mam- 

 illaris, tractus olfacto-corticalis, etc. The tertiary olfactory 

 centers into which these tracts of the third order discharge 

 are arranged in two series: (1) the basal centers of the dien- 

 cephalon and cerebral peduncle for olfactory reflexes, and (2) 

 the cortical centers in the hippocampus and gyrus hippocampi. 



122. Peripheral olfactory organ. — If microscopic sections of 

 the nasal epithelium are available, they should be studied at 

 this time. Note that this sensory epithelium differs histologic- 

 ally in important respects from any other in the human body. 

 See Barker ('01), Fig. 208; Herrick ('15), Figs. 36, 103, and 104; 

 Quain ('09), Figs. 70 and 71; Sheldon ('18), Figs. 333 and 352. 



123. Olfactory tracts of the sheep. — The olfactory fibers of the 

 second order arise from the mitral cells of the olfactory bulb 

 and form the tractus olfactorius, of which there are three 

 parts. For their arrangements in the sheep see Fig. 8. 



(1) Stria olfactoria lateralis (radix lateralis bulbi olfactorii). — 

 This can easily be dissected out, following the fissura rhinalis 

 from the olfactory bulb to the tip of the gyrus hippocampi 

 (lobus hippocampi, lobus piriformis). In the human brain it 

 runs farther lateralward in the lateral fissure to the border of 

 the insula and then bends sharply medialward and backward 

 to enter the uncus of the temporal lobe. 



(2) Stria olfactoria medialis (radix medialis bulbi olfactorii). — 

 This can be dissected out and will be found to ascend on the 

 median surface of the hemisphere and to terminate chiefly in the 

 medial olfactory area under the genu of the corpus callosum. 

 This area includes the gyrus subcallosus and septum. 



(3) Stria olfactoria intermedia. — This lies between the medial 

 and lateral striae. Part of it can be dissected out directly into 

 the anterior commissure (Burkholder ('12), Plate XIX), 

 within which it decussates to terminate in the anterior perfor- 

 ated space (tuberculum olfactorium) of the opposite hemisphere. 

 This tract can best be dissected by locating the anterior com- 



