ABDUCENS NERVE. 693 



trophic relations of the nerves to the tissues, is the rare condition of unilateral 

 hypertrophy of the face, which bears some resemblance to the analogous manifesta- 

 tions of so-called partial giant-growth (acromegaly) . 



Reference should be made here also to the extremely remarkable observation 

 of Urbantschitsch, who found that irritation of branches of the trigeminus, and 

 principally those that pass to the ear, causes increase in the light-sense of the 

 individual in question. Blowing on the cheek or the nasal mucous membrane, 

 electrical irritation, the snuffing of tobacco, the smelling of strong odors may tem- 

 porarily increase the sensibility to light. Also the gustatory and the olfactory 

 sense, as well as the sensibility of certain cutaneous areas, may be thus increased 

 reflexly through slight irritation of the trigeminus. In the presence of severe 

 affections of the ear, in consequence of which fibers of the trigeminus may be 

 seriously involved, the sense-functions mentioned may be impaired. Local im- 

 provement in the aural disorder is then attended with an increase in the activity 

 of the special senses mentioned. 



After extirpation of the Gasserian ganglion, together with its roots, in man 

 the entire distribution of the trigeminus has been found completely and irremedi- 

 ably anesthetic. All parts remained intact so far as their trophic state was con- 

 cerned, but the anesthetic eye was less resistant to influences exciting inflamma- 

 tion, and Keen and Laguaite observed keratitis after extirpation of the ganglion, 

 and Seheier ulceration of the cornea and the mucous membrane of the mouth 

 and the nose after injury to the nerve. The secretion of tears was in some in- 

 stances diminished, in others abolished. The skin of the cheek and of the eyebrow 

 exhibited slight trophic change. Immediately after the operation the skin ex- 

 hibited signs of abnormal distribution of the blood and later a sense of heat on 

 the forehead and in the eye. The sense of taste was impaired in the distribution 

 of the lingual nerve and likewise that of smell in the corresponding nasal cavity. 

 The muscles of mastication are paralyzed, and the delicacy of movement in the 

 muscles of the face was impaired in consequence of absence of the muscle-sense. 

 In the course of time the anesthetic area becomes smaller, as branches from adja- 

 cent nerves grow into it. 



VI. ABDUCENS NERVE. 



The abducens nerve arises from the abducens nucleus with neurites from 

 large cells that correspond to those of the anterior horns of the spinal cord. The 

 nucleus lies below the eminentia teres on the floor of the fourth ventricle (Fig. 241) 

 and below the knee-shaped flexure of the facial nerve. Probably some oculo- 

 motor fibers arise from the abducens nucleus, and from the left those fibers of 

 the right oculomotor that rotate the right eye inward (this explains the synergistic 

 action of the two eyes in lateral movement). Physiologically, connecting fibers 

 should pass between the nucleus of origin and the contralateral cortical center in 

 the cerebrum for the ocular movements. The nerve makes its appearance at the 

 posterior margin of the pons (Fig. 242). 



The abducens is the voluntary nerve of the external rectus muscle, 

 although in the coordinated movements of the eye it is stimulated in- 

 voluntarily. 



Branches of considerable size pass from the sympathetic in the cavernous 

 sinus to the abducens nerve (Fig. 243, 6); smaller branches from the trigeminus, 

 whose significance is the same as that of analogous branches of the trochlear and 

 oculomotor. 



Pathological. Complete paralysis causes internal strabismus, and as a result 

 diplopia. In dogs, division of the cervical sympathetic causes slight rotation 

 of the eyeball inward. This must be due to the fact that the abducens receives 

 motor muscle-nerves from the cervical sympathetic. Spasm of the abducens 

 causes external strabismus. 



With reference to strabismus, it should be mentioned that it may be caused, 

 in addition to irritation or paralysis of the nerves, also by primary muscular 

 affections, such as congenital shortness, contractures, injuries. Finally, strabis- 

 mus occurs in connection with cloudiness of the transparent media of the eye, 

 the individual involuntarily rotating the eye so that the visual rays, so far as 

 possible, pass through the portions of the media that are still clear. Lesions of 

 the retina at the yellow spot give rise to similar results. The affected eye, further, 



