104 LABORATORY OUTLINE OF NEUROLOGY 



121. Rhinencephalon. The entire olfactory part of the brain 

 is called the rhinencephalon. This apparatus is so much more 

 highly developed in the sheep than in man that the dissection 

 is much more readily carried out upon this brain. Before 

 undertaking the following dissection look up in the reference 

 books the structure of the olfactory epithelium, nerve and bulb. 

 See Cunningham (15), p. 623; Herrick ('18), Chap. XV; 

 Howell ('18), pp. 299-305; Villiger ('12), Fig. 116, p. 118. 

 On the nervus terminalis see Section 47 (d). 



The peripheral olfactory neurons arise from cells lying in 

 the mucous membrane of the nose. These fibers terminate in 

 the olfactory bulb, which is the primary olfactory center of the 

 brain. Here lie the neurons of the second order (mitral cells), 

 whose axons constitute the olfactory tracts, or striae, terminat- 

 ing in secondary olfactory centers in the basal parts of the 

 cerebral hemisphere. 



These secondary centers in the aggregate are called the area 

 olfactoria and the fibers of the second order terminating in 

 them are called tractus olfactorius (lateralis, medialis, and in- 

 termedius). 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-mamil- 

 laris, 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 gyms 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 ('18), Figs. 36, 103, and 104; 

 Piersol (16), Figs. 1179, 1180, p. 1415; Quain ( ; 09), Figs. 70 

 and 71. 



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 arrangement in the sheep see Fig. 8. 



