THE CEREBRAL NERVES 



357 



the viscera and blood vessels (spinal nerves) ; or (6) with fibers ending 

 directly on visceral muscle fibers (mixed cerebral nerves). 



B. THE CEREBRAL NERVES 



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

 differ from the spinal nerves: (i) in that they are not segmentally ar- 

 ranged, and (2) in that they do not all contain the same types of nervous 

 components. Classed according to the functions of their neurons they 

 fall into three groups: 



VISCERAL SENSORY AND 

 SOMATIC MOTOR MOTOR. 



III. Oculomotor. V. Trigeminal. 



IV. Trochlear. VII. Facial. 



VI. Abducens. IX. Glossopharyngeal. 



SPECIAL SOMATIC SENSORY. 

 I. Olfactory. 

 II. Optic. 

 VIII. Acoustic. 



XII. Hypoglossal. 



X. Vagus complex, including 

 XI. Spinal Accessory. 



It will be seen: (i) that the nerves of the first group are purely sen- 

 sory, corresponding to the general somatic afferent neurons of the spinal 

 nerves; (2) that those of the somatic motor group are purely motor and 

 correspond to the somatic efferent neurons of the spinal nerves; (3) that 

 those of the third group are mixed in function and correspond to the vis- 

 ceral components of the spinal nerves. 



I. THE SPECIAL SOMATIC SENSORY NERVES 



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

 nerve cells lie at first in the olfactory epithelium of the nose and are of the 

 bipolar type (fourth week). From 

 these cells peripheral processes de- 

 velop and end directly at the surface 

 of the olfactory epithelium (Fig. 

 361). Central processes grow to- 

 ward 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 neurons of the second 

 order. Some olfactory cells migrate 



from the epithelium, with which, however, they retain peripheral con- 

 nections. Such bipolar cells, found along the entire course of the nerve, 

 resemble ordinary dorsal ganglion cells. The olfactory nerve fibers are 

 peculiar in that they remain unmyelinated. Nerve fibers from the 



"Olfactory tract 

 -Mitral cell 



Glomerulus 



-Cribriform plate 

 -Olfactory nerve fiber 



\~Olfactory epithelium 



FIG. 361. Diagram of the relations of the 

 fibers in the olfactory nerve. 



