FIFTH PAIR. 451 



its division has the effect of paralyzing the muscles to which it is 

 distributed. The superior eyelid falls down over the pupil, and 

 cannot be raised, owing to the inaction of its levator muscle, so 

 that the eye appears constantly half shut. This condition is known 

 by the name of " ptosis." The movements of the eyeball are also 

 nearly suspended, and permanent external strabismus takes place, 

 owing to the paralysis of the internal rectus muscle, while the ex- 

 ternal rectus, animated by a different nerve, preserves its activity. 



PATHETIC as. This nerve, which supplies the superior oblique 

 muscle of the eyeball, is similar in its general properties to the pre- 

 ceding. Its section causes paralysis of the above muscle, "without 

 any loss of sensibility. 



MOTOR EXTERNUS. This nerve, the sixth pair, according to the 

 usual anatomical arrangement, is distributed to the external rectus 

 muscle of the eyebalL Its division or injury by disease is followed 

 by internal strabismus, owing to the unopposed action of the internal 

 rectus muscle. 



FIFTH PAIR. This is one of the most important and remarkable 

 in its properties of all the cranial nerves. It is the great sensitive 

 nerve of the face, and of the adjoining mucous membranes. Its 

 larger root, after emerging from the outer and under surface of the 

 pons Varolii, passes forward over the inner extremity of the petrous 

 portion of the temporal bone. It there expands into a crescentic- 

 shaped swelling, containing a quantity of gray matter with which 

 its fibres are intermingled, and which is known as the Casserian 

 ganglion. The fibres of the smaller root, passing forward in com- 

 pany with the others, do not take any part in the formation of this 

 ganglion, but may be seen passing beneath it as a distinct bundle, 

 and continuing their course forward to the foramen ovale, through 

 which they emerge from the skull. In front of the anterior and 

 external border of the Casserian ganglion, the fifth nerve separates 

 into three principal divisions, viz., the ophthalmic, the superior 

 maxillary, and the inferior maxillary. The first of these divisions, 

 viz., the ophthalmic, is so called because it passes through the orbit 

 of the eye. It enters the sphenoidal fissure, and runs along the 

 upper portion of the orbit, sending branches to the ophthalmic gan- 

 glion of the sympathetic, to the lachrymal gland, the conjunctiva. 



