6 2 6 TEXT-BOOK OF PHYSIOLOGY 



eyes, face, the respiratory and cardiac mechanisms, which indicate that the 

 afferent fibers are centrally in relation with a number of motor nerve- 

 centers. 



THE SIXTH NERVE. THE ABDUCENT 



The sixth cranial nerve, the abducent, consists of peripherally coursing 

 axons which serve to bring the nerve-cells from which they arise into rela- 

 tion with the external rectus muscle. 



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 prob- 

 able 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 med- 

 ulla oblongata and the 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. In its course the nerve 

 receives filaments from the carotid plexus of the sympathetic. 



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 cms 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 con- 

 traction of the external rectus muscle and external deviation of the eyeball. 

 Division of the nerve is followed by paralysis or relaxation 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 and parallel. The image of the paralyzed eye lies 

 external to that of the normal eye. 



Function. The function of this nerve is to transmit nerve impulses to 

 the external rectus muscle and excite it to contraction. 



THE SEVENTH NERVE. THE FACIAL 



The seventh cranial nerve, the facial, consists of peripherally coursing 

 nerve-fibers, which serve to bring the nerve-cells from which they arise 

 into relation with most of the superficial muscles of the head and face. 



The muscles supplied by this nerve, as stated by the general anatomists, 

 are as follows: The occipito-frontalis, corrugator supercilli, orbicularis 

 palpebrarum, levator labii superioris alaeque nasi, zygomatici, the pyramidalis 

 nasi, compressor nasi, depressor alae nasi, levator anguli oris, buccinator, 

 orbicularis oris, depressor anguli oris, depressor labii inferioris, levator 

 menti, posterior belly of the digastric, stylohyoid, and platysma myoides. 

 These muscles by their individual and cooperative contraction express ideas 

 and feelings and are therefore termed muscles of expression. 



Origin. The nerve-fibers or axons composing the seventh nerve arise 

 for the most part from a nucleus of large multipolar nerve-cells situated 



