542 



TEXT-BOOK OF PHYSIOLOGY. 



Through the relation of its central end-tufts with the motor nuclei in 

 the medulla and pons, it assists in the reflex acts of mastication and 

 insalivation. 



SIXTH PAIR. THE ABDUCENS. 



The sixth cranial nerve, the abducens, consists of peripherally 

 coursing axons which serve to bring the nerve-cells from which they 

 arise into relation with the external rectus muscle. 

 | j Origin. The axons arise from a group of cells located in the 

 gray matter beneath the upper half of the floor of the fourth ventricle. 



It is quite probable that a few 

 fibers in each nerve- trunk come 

 from the nucleus on the opposite 

 side of the middle line. 



Distribution. The nerve- 

 fibers pass forward from their 

 origin through the gray and 

 white matter and emerge through 

 the groove between the medulla 

 oblongata and me pons Varolii 

 just external to the anterior 

 pyramid. The nerve then passes 

 through the sphenoid fissure into 

 the orbit cavity, where it is 

 distributed to the external rectus 

 muscle (Fig. 251). In its course 

 the nerve receives filaments from 

 the carotid plexus of the sym- 

 pathetic. 



Cortical Connections. 

 The nucleus of the sixth nerve 

 is in histologic and physiologic 

 connection with the motor area 

 of the cerebral cortex. From 



nerve-cells in this region axons are given off which enter the 

 pyramidal tract, descend through the internal capsule and crus 

 cerebri, after which they cross to the opposite side, where their 

 end-tufts arborize around the cells of the nucleus already described. 

 Properties. Stimulation of the nerve is followed by spasmodic 

 contraction of the external rectus muscle and external deviation of 

 the eyeball. Division of the nerve is followed by paralysis or relaxa- 

 tion of the muscle. As a result of the unopposed action of the 

 internal rectus the anterior pole of the eyeball is turned toward the 

 middle line (internal strabismus). In consequence of this deviation 

 there is homonymous diplopia. The images are on the same level 



FIG. 251. DISTRIBUTION or THE MOTOR 



OCULI EXTERNUS OR ABDUCENS. I. 



Trunk of the motor oculi communis, 

 with its branches (2, 3, 4, 5, 6, 7). 8. 

 Motor oculi externus, passing to 

 the external rectus muscle. 9. Fila- 

 ments of the motor oculi externus 

 anastomosing with the sympathetic. 

 10. Ciliary nerves. (Hirschfeld.) 



