102 HUMAN PHYSIOLOGY. 



CRANIAL NERVES. 



The Cranial Nerves come off from the base of the brain, pass through 

 the foramina in the walls of the cranium, and are distributed to the skin, 

 muscles and organs of sense in the face and head. 



According to the classification of Soemmering, there are 12 pairs of 

 nerves, enumerating them from before backward, as follows, viz : 



1st Pair, or Olfactory. 7th Pair, or Facial, Portio dura. 



2d Pair, or Optic 8th Pair, or Auditory, Portio mollis. 



3d Pair, or Motor oculi communis. Qth Pair, or Glosso-pharyngeal. 



4th Pair, or Patheticus, Trochlearis. loth Pair, or Pneumogastric. 



5th Pair, or Trifacial, Trigeminus. I ith Pair, or Spinal accessory. 



6th Pair, or Abducens. 1 2th Pair, or Hypoglossal. 



The Cranial Nerves may also be classified physiologically, according 

 to their function, into three groups : I. Nerves of special sense. 2. Nerves 

 of motion. 3. Nerves of general sensibility. 



ist Pair. Olfactory. 



Apparent Origin. From the inferior and internal portion of the ante- 

 rior lobes of the cerebrum by three roots, viz : an external white root, which 

 passes across the fissure of Sylvius to the middle lobe of the cerebrum ; an 

 internal white root, from the most posterior part of the anterior lobe ; &gray 

 root, from the gray matter in the posterior and inner portion of the inferior 

 surface of the anterior lobe. 



Deep Origin. Not satisfactorily determined. 



Distribution. The olfactory nerve, formed by the union of the three 

 roots, passes forward along the under surface of the anterior lobe to the 

 ethmoid bone, where it expands into the olfactory bulb. This bulb con- 

 tains ganglionic cells, is grayish in color and soft in consistence ; it gives off 

 from its under surface from fifteen to twenty nerve filaments, the true 

 olfactory nerves, which pass through the cribriform plate of the ethmoid 

 bone, and are distributed to the schneiderian mucous membrane. This 

 membrane extends from the cribriform plate of the ethmoid bone downward, 

 about one inch. 



Properties. The olfactory nerves give rise to neither motor nor sensory 

 phenomena when stimulated. They carry simply the special impressions 

 of odorous substances. Destruction or injury of the olfactory bulbs is 

 attended by a loss of the sense of smell. 



Function. Governs the sense of smell. Conducts the impressions which 

 give rise to odorous sensations. 



