CHAPTER XXIII. 

 THE ENCEPHALIC OR CRANIAL NERVES. 



The nerve-trunks which serve as channels of communication between the 

 encephalon and the structures of the head, the face, and in part the organs 

 of the thorax and abdomen, pass through foramina in the walls of the cranium, 

 and for this reason are termed cranial nerves. 



According to the classification now generally adopted, there are twelve 

 cranial nerves on either side of the median line, which, enumerated from 

 before backward, are as follows (Fig. 262) : 



First or Olfactory. Seventh or Facial. 



Second or Optic. Eighth or Acoustic. 



Third or Oculo-motor. Ninth or Glosso-pharyngeal. 



Fourth or Trochlear. Tenth or Pneumogastric or Vagus. 



Fifth or Trigeminal. Eleventh or Spinal Accessory. 



Sixth or Abducent. Twelfth or Hypoglossal. 



The cranial nerves may be classified physiologically in accordance with their 

 functional manifestations into three groups, viz. : 



1. Nerves of Special Sense: e.g., Olfactory, Optic, Acoustic, Gustatory (Glosso-pharyngeal) 



2. Nerves of General Sensibility: e.g., Large root of the Trigeminal, Glosso-pharyngeal, and 

 Pneumogastric. 



3. Nerves of Motion: e.g., Oculo-motor, Trochlear, the small root of the Trigeminal, Abducent, 

 Facial, Spinal Accessory, and Hypoglossal. 



Though this classification in the main holds true, it must be borne in mind 

 that modern investigations have demonstrated that the glosso-pharyngeal 

 and pneumogastric nerves contain even at their junction with the medulla 

 oblongata a number of efferent or motor fibers, and to this extent are mixed 

 nerves. 



The Origins of the Cranial Nerves. In accordance with modern 

 views as to the origins of nerves in general, it may be stated that 



The nerves of special sense have their origin respectively in the neuro- 

 epithelial cells in the mucous membrane of the olfactory region of the nose, 

 in the ganglion cells of the retina, in the cells of the spiral ganglion of the 

 cochlea and the ganglion of Scarpa, and in the cells of the petrous and jugular 

 ganglia. From the cells of these ganglia dendrites pass peripherally to 

 become associated with specialized end-organs, while axons pass centrally 

 in well-defined bundles to become related by means of their end-tufts with 

 primary basal ganglia. 



The nerves of general sensibility have their origin in the ganglia on their 

 trunks, and in this respect resemble the spinal nerves. From the ganglion 

 cell there emerges a short axon process which soon divides into a central and a 

 peripheral branch. The former passes toward and into the gray matter 

 located beneath the floor of the fourth ventricle, where its end-tufts arborize 

 about nerve-cells. The latter (the peripheral branch) passes toward the 

 general periphery to be distributed to skin and mucous membranes. 



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