THE CEREBRAL NERVES 



357 



B. THE CEREBRAL NERVES 



The cerebral nerves of the human brain are twelve in number. They differ 

 from the spinal nerves: (1) in that they are not segmen tally arranged, and (2) 

 in that they do not all contain the same types of nervous components. Classed 

 according to the functions of their neurones they fall into three groups : 



Special Somatic Sensory. 

 I. Olfactory. 

 II. Optic. 

 VIII. Acoustic. 



Somatic Motor or Efferent. 



III. Oculomotor. 



IV. Trochlear. 

 VI. Abducens. 



XII. Hypoglossal. 



Visceral Sensory and Motor. 

 v. Trigeminal. 

 VII. Facial. 

 IX. Glossopharyngeal. 

 X. Vagus complex, including 

 XI. Spinal Accessory. 



It wiU be seen (1) that the nerves of the first group are purely sensory, 

 corresponding to the general somatic afferent neurones of the spinal nerves; (2) 

 that the nerves of the somatic motor group are purely motor and correspond to the 

 somatic efferent or motor neurones of the spinal nerves ; (3) that the nerves of the 

 third group are of mixed function and correspond to the visceral components of 

 the spinal nerves. 



L The Special Somatic Sensory Nerves 

 1. The Olfactory Nerve, though purely sensory, has no ganglion. Its nerve 



cells He at first in the olfactory epithelium of the nose and are of the bipolar type 



(fourth week) . From these cells periph- 

 eral processes develop and end directly 



at the surface of the olfactory epithe- 



Hum (I'ig. 361). Central processes grow 



toward the olfactory lobe and form the 



strands of the olfactory nerve. They end 



in the glomeruli of the olfactory bulb in 



contact with the dendrites of the mitral 



cells, or olfactory neurones of the second 



order. Some olfactory cells migrate 



from the epithelium, with which, however, they retain peripheral connections. 



Such bipolar cells, found along the entire course of the nerve, resemble ordinary 



dorsal gangUon cells. The olfactory nerve fibers are pecuhar in that they remain 



unmyehnated. Nerve fibers from the epithelium of the vestigial vomer o-nasal 



organ (of Jacobson) also end in the olfactory bulb. 



When the ethmoidal bone of the cranium is developed, its cartilage, as the 



cribriform plate, forms around the strands of the olfactory nerve. 



Olfactory tract 

 —Mitral cell 



■ — Glomerulus 



Cribriform plate 

 Olfactory nerve fiber 



{ ■-Olfactory epithelium 

 trwsf I ' ' — 1 — r— 1 — ( — I — I — f — (— * 



Fig. 361.— Diagram of the relations of the 



fibers in the olfactory nerve. 



