THE BRAIN. 513 



Cranial Nerves (Fig. 303). The cranial nerves have their 

 origin in the gray matter at the base of the brain, and they escape 

 from the skull by various openings, or foramina, to reach the parts 

 to which they are distributed. The only exception to this is the 

 spinal accessory, a part of which arises from the gray matter of 

 the cord. Among the cranial nerves are those of special sense, 

 of motion, and nerves having both motor and sensory properties. 

 The points at which they leave the brain are spoken of as their 

 apparent origin, but this is only apparent, for they can be traced 

 into the brain-substance, to collections of nerve-cells, nerve- 

 centers, to which the name nuclei has been given. The nucleus 

 of a nerve is its real origin. 



Of cranial nerves there are 12 pairs, the number of each indicat- 

 ing the order, from before backward, in which it escapes from the 

 cavity of the cranium: 1. Olfactory; 2. Optic; 3. Motor oculi 

 communis ; 4. Patheticus or trochlearis ; 5. Trigeminus ; 6. Ab- 

 ducens ; 7. Facial ; 8. Auditory ; 9. Glossopharyngeal ; 10. Pneu- 

 mogastric; 11. Spinal accessory ; 12. Hypoglossal. 



The first two nerves, the olfactory and the optic, will be 

 considered in connection with the senses of smell and sight. 



Motor Oculi. The third nerve, motor oculi, motor oculi com- 

 munis, or oculomotorius, leaves the surface of the brain at the inner 

 surface of the crus cerebri, just in front of the pons Varolii. Its real 

 origin is, however, a nucleus in the floor of the aqueduct of Sylvius. 

 It escapes from the cranium through the sphenoidal fissure, and is 

 distributed to the superior, internal, and inferior recti and to the 

 inferior oblique. It also supplies the levator palpebrae superioris, 

 and sends a branch to the ophthalmic, lenticular, or ciliary gang- 

 lion. Another way to describe its distribution is to say that it 

 supplies the levator palpebrse and all the muscles that move the 

 eyeball, except the superior oblique and external rectus. 



The action of these muscles is largely indicated by their names. 

 The levator palpebne by its contraction raises the upper eyelid. 

 The internal rectus turns the eyeball inward toward the nose, and 

 the external rectus turns it outward. The direction of action and the 

 point of attachment of the superior rectus are such that when it con- 

 tracts the eyeball is not only turned upward, but it is also rotated 

 slightly inward ; this is corrected by the action of the inferior 

 oblique, so that the two acting together produce a movement 

 directly upward. The same deviation inward follows when the 

 eye is turned downward by the inferior rectus, and a similar cor- 

 rection is made by the action of the superior oblique. If the 

 external and superior recti act together, the movement of the 

 eyeball is in the direction of the diagonal that is, outward and 

 upward ; the conjoint action of the external and inferior recti 

 causes the eyeball to move outward and downward, and a corre- 

 sponding action results when the other adjacent recti are brought 



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