THE CRANIAL NERVES. 



525 



motor region of the cortex through descending axons contained in the 

 pyramidal tract. The end-tufts of these axons arborize around the 

 nerve- cells. The efferent path beginning in the cerebral cortex is 

 thus continued by the motor nerves to the general periphery. 



The three groups of nerves, those of special sense, of general 

 sensibility, and the motor nerves, are neurons of the first order; the 

 nerve-cells and fibers which constitute the cerebral connections are 

 neurons of the second order. It is possible that the sensor cells 

 in the cerebral cortex are neurons of a third order. 



FIRST PAIR. THE OLFACTORY. 



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

 third of the nasal fossa, in the regio oljactoria. It consists of from 

 20 to 30 branches, the fibers of which are non-medullated. 



Origin. The olfactory nerve is composed of rpntrally coursing 

 axojis which have their 

 origin in the central ends 

 ot Dipolar, rod-shaped7~br 

 spindle-shaped nerye^cells 

 interspersed among the epi- 

 thelial cells covering the 

 mucous membrane in the 

 regio olfactoria ; the__j)er- 

 ipheral ends. of .the*se,xells 

 give off a number of den- 

 drites which are spread out 

 to form a delicate feltwdrk 

 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 

 foraminaSi which they'pass 

 to oecome related by their 



FIG. 238. THE RELATION OF THE OLFACTORY 



NERVES TO THE OLFACTORY TRACT, i. 

 Olfactory nerve-cell. 2. Axon process. 3. 

 Epithelial cells. 4. Glomerulus. 5. Mitral 

 cells. 6. Centrally coursing axons of the 

 olfactory tract. 



enclitufts with structures in 

 the gray matter of the 



"Corli^lConnections. 



The olfactory bulb and 



olfactory tract, formerly called the olfactory nerve, are portions of 

 the cerebrum (the olfactory lobe) which arise embryologically by a 

 protrusion of the walls of the cerebral cavity. The bulb is oval- " 



