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TEXT-BOOK OF PHYSIOLOGY. 



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 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 mid- 

 dle line (internal strabismus). In con- 

 sequence 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. 



SEVENTH NERVE. THE FACIAL. 



FIG. 276. DISTRIBUTION OF 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 exter- 

 nal rectus muscle. 9. Filaments of the 

 motor oculi externus anastomosing with 

 the sympathetic. 10. Ciliary nerves. 

 (Hirschfeld.) 



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, cor- 

 rugator supercilii, 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, de- 

 pressor labii inferioris, levator menti, posterior belly of the digastric, stylo- 

 hyoid, and platysma myoides. 



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

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

 about five millimeters beneath the upper half of the floor of the fourth ven- 

 tricle toward the middle line. 



From this nucleus, which is about four millimeters long, axons emerge 

 which at first pass inward and backward as far as the ependyma of the ven- 

 tricle; they then turn on themselves, forming an arch that encloses the nu- 

 cleus of the sixth nerve; they then course downward and outward, emerging 

 from the pons at its lower border between the olivary and restiform bodies. 

 As the axons approach the floor of the ventricle collateral branches are 

 given off which, crossing the median line, arborize around the nerve-cells 

 of the opposite facial nucleus. 



Clinic observations and histologic investigations, however, render it 

 probable that the fibers distributed to the occipito-frontalis, the corrugator 



