THE CEREBRAL NERVES 355 



fibers (mixed cerebral nerves). The relation of the sympathetic system to the 

 central nervous system is described on page 364. 



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 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 SOMATIC MOTOR VISCERAL SENSORY 



SENSORY OR EFFECTOR AND MOTOR 



I. Olfactory. III. Oculomotor. V. Trigeminal. 



II. Optic. IV. Trochlear. VII. Facial. 



VIII. Acoustic. VT. Abducens. IX. Glossopharyngeal. 



XII. Hypoglossal. X. Vagus complex including 



XI. Spinal Accessory. 



It will be seen (i) 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. 



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. 

 From these cells peripheral processes develop and end directly at the surface of the 

 olfactory epithelium. Central processes grow into contact with the olfactory 

 lobe and form the strands of the olfactory nerve. They end in the olfactory 

 bulb about the peculiar mitral cells. Some of the olfactory cells migrate from 

 epithelium along with the developing nerve fibers, and may be found as bipolar 

 cells along the course of the nerve. The olfactory nerve fibers are peculiar in 

 that they remain non-medullated. 



When the ethmoidal bone of the cranium is developed its cartilage is formed 

 around the strands of the olfactory nerve, which thus in the adult penetrate the 

 cribriform plate of the ethmoid. 



Nerve fibers which pass from the epithelium of the organ of Jacobson also 

 end in the olfactory bulb. The organ of Jacobson is a vestigial sense organ and 

 its nerve is rudimentary. For the development of the olfactory organ see p. 369. 



