THE CRANIAL NERVES. 



573 



The nerves of motion have their origin in the nerve-cells in the gray matter 

 beneath the aqueduct of Sylvius and beneath the floor of the fourth ventricle. 

 The axons emerging from these cells course peripherally to be distributed 

 to skeletal muscles. In some of the motor nerves, and in some sensor 

 nerves as well, there are to be found efferent fibers of smaller size which 

 have a similar origin and which become related through the intervention 

 of sympathetic ganglia (peripheral neurons) with visceral muscles and 

 glands. These nerves have been termed autonomic nerves. 



The Cortical Connections of the Cranial Nerves. Each of these three 

 groups of cranial nerves has special connections with the cerebral cortex. 



The nerves of special sense for the 

 most part terminate in primary basal 

 ganglia, around the cells of which their 

 central end- tufts arborize. From these 

 cells axons arise which pass upward and 

 directly or indirectly come into physio- 

 logic relation with sensor nerve-cells in 

 the cerebral cortex. 



The nerves of general sensibility ter- 

 minate in the gray matter beneath the 

 floor of the fourth ventricle, around the 

 nerve-cells of which their end-tufts arbor- 

 ize. These groups of nerve-cells are 

 known as sensor end-nuclei. Though 

 once regarded as the centers of origin of 

 the sensor nerves, they are now regarded 

 as the centers of origin of axons which 

 pass upward to the cortex of the cere- 

 brum, where they also come into physio- 

 logic relation with sensor nerve-cells. 



The axons in both of these classes of 

 nerves thus originate in the cells of the 

 central nerve system and continue up- 

 ward to the cerebrum, the primary affer- 

 ent path. 



The motor nerves which have their 

 origin ' in the cells of the gray matter 

 beneath the aqueduct of Sylvius and beneath the floor of the fourth ventricle 

 are in physiologic relation with nerve-cells in the motor region of the 

 cortex through descending axons contained in the pyramidal tract, the end- 

 tufts of which arborize around the nerve-cells. The efferent path be- 

 ginning in the cerebral cortex is thus continued by the motor nerves to the 

 general periphery. 



The three groups of nerves, those of special sense, of general sensibility, 

 and the motor nerves, are neurons of the first order; the nerve-cells and 

 fibers which constitute the cerebral connections are neurons of the second 

 order. It is probable that the sensor cells in the cerebral cortex are neurons 

 of a third order. 



FIG. 262. SUPERFICIAL ORIGIN OF 

 THE CRANIAL NERVES FROM THE BASE 

 OF THE ENCEPHALON. i. Olfactory. 2. 

 Optic. 3. Motor oculi. 4. Trochlear. 

 5. Trigeminal. 6. Abducent 7. Facial. 

 7'. Nerve of Wrisberg. 8. Acoustic. 9. 

 Glosso-pharyngeal. 10. Pneumogastric. 

 n. Spinal accessory. 12. Hypoglossal. 

 (Moral and Doyon,) 



