THE ENCEPHALIC OR CRANIAL NERVES 



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THE FIRST NERVE. THE OLFACTORY 



The first cranial nerve, the olfactory, is situated in the upper third of the 

 nasal fossa, in the regio olfactoria. It consists of from 20 to 30 branches, the 

 fibers of which are non-medullated. 



Origin. The olfactory nerve is composed of centrally coursing axons 

 which have their origin in the central ends of bipolar, rod-shaped, or spindle- 

 shaped nerve-cells interspersed among the epithelial cells covering the mucous 

 membrane in the regio olfactoria; the peripheral ends of these cells give off 

 a number of dendrites which are spread out to form a delicate feltwork over 

 the surface of the mucous membrane. From their origin the axons gradually 

 converge to form bundles which 

 ascend to the cribriform plate of 

 the ethmoid bone, through the 

 foramina of which they pass to be- 

 come related by their end-tufts with 

 structures in the gray matter of 

 the olfactory bulb (Fig. 258). 



Cortical Connections. The 

 olfactory bulb and olfactory tract, 

 formerly called the olfactory nerve, 

 are portions of the cerebrum (the 

 olfactory lobe) which arise em- 

 bryologically by a protrusion of 

 the walls of the cerebral cavity. 

 The bulb is oval-shaped and con- 

 sists of both gray and white matter. 

 It rests on the cribriform plate of 

 the ethmoid bone and is embraced 

 by the olfactory nerves. As seen 

 on sagittal section, there is just be- 

 neath the surface a layer of large 

 pyramidal and spindle-shaped 

 cells (termed also mitral cells), 

 each provided with an apical and 

 two lateral dendrites. The apical dendrite passes toward the surface and 

 ends in a brush- or basket-like expansion which interlaces with the end- 

 tufts of the olfactory nerves, forming what are known as the olfactory glom- 

 erules. The lateral dendrites end free. 



The axons of the pyramidal cells pass toward the center of the bulb and 

 bend at right angles, after which they pursue a horizontal direction toward 

 and into the olfactory tract. This tract is about five centimeters in length, 

 prismatic in shape on cross-section and divisible into a. ventral and a dorsal 

 portion. It emerges from the posterior extremity of the bulb, passes back- 

 ward to the posterior part of the anterior lobe, where it divides into three 

 roots: viz., a lateral or external, a mesial or internal, a middle or dorsal. 

 The fibers of the lateral and mesial roots are derived almost exclusively 

 from the ventral portion of the tract, the fibers of which come from the 

 mitral cells in the bulb. The lateral root-fibers pass outward into the fossa of 

 Sylvius and come into relation with nerve-cells in the inferior extremity of 

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FIG. 258. THE RELATION OF THE OLFACTORY 

 NERVES TO THE OLFACTORY TRACT, i. Ol- 

 factory nerve-ceil. 2. Axon process. 3. Epi- 

 thelial cells. 4. Glomerulus. 5. Mitral cells. 

 6. Centrally coursing axons of the olfactory 

 tract. (Morat and Doyon.) 



