724 



THE NERVOUS SYSTEM. 



orbit through the sphenoidal fissure, between the two heads of the External rectus 

 muscle. On passing through the fissure, the nerve is placed below the fourth and 

 the frontal and lachrymal branches of the ophthalmic nerve, and has passing be- 

 tween its tw r o divisions the nasal nerve. 



The superior division, the smaller, passes inward over the optic nerve, and 

 supplies the Superior rectus and Levator palpebme. 



The inferior division, the larger, divides into three branches. One passes 

 beneath the optic nerve to the Internal rectns ; another, to the Inferior rectus ; 

 and the third, the longest of the three, passes forward between the Inferior and 

 External recti to the Inferior oblique. From this latter a short, thick branch is 

 given off to the lower part of the lenticular ganglion, which forms its inferior 

 root. It also gives oft' one or two filaments to the Inferior rectus. All these 

 branches enter the muscles on their ocular surface, except that to the Inferior 

 oblique, which enters its posterior border. 



Surgical Anatomy. Paralysis of the third nerve may be the result of many causes : as 

 cerebral disease ; conditions causing pressure on the cavernous sinus ; periostitis of the bones 

 entering into the formation of the sphenoidal fissure. It results, when complete, in (1) ptosis, 

 or drooping of the upper eyelid, in consequence of the Levator palpebrse being paralyzed ; (2) 

 external strabismus, on account of the unopposed action of the External rectus muscle, which 

 is not supplied by the third nerve, and is not therefore paralyzed; (3) dilatation of the pupil, 

 because the sphincter fibres of the iris are paralyzed; (4) loss of power of accommodation, as 

 the sphincter pupillse, the ciliary muscle, and the Internal rectus are paralyzed ; (5) slight 

 prominence of the eyeball, owing to most of its muscles being relaxed. Occasionally paralysis 

 may affect only a part of the nerve ; that is to say, there may be, for example, a dilated and 

 fixed pupil, with ptosis, but no other signs. Irritation of the nerve causes spasm of one or otlicr 

 of the muscles supplied by it ; thus, there may be internal strabismus from spasm of the Internal 

 rectus ; accommodation for near objects only from spasm of the ciliary muscle, or myosis, contrac- 

 tion of the pupil, from irritation of the spliincter of the pupil. 



The Fourth Nerve (Fig. 391). 



The Fourth or Trochlear nerve (n. trocJilearis), the smallest of the cranial 

 nerves, supplies the Superior oblique muscle. 



Its apparent origin, at the base of the brain, is on the outer side of the crus 

 cerebri, just in front of the pons Varolii, but the fibres can be traced backward 



FIG. 392. Plan of the motor oculi nerve. (After Flower.) 



behind the corpora quadrigemina to the valve of Vieussens, on the upper surface 

 of which the two nerves decussate. Its deep origin may be traced to a nucleus 

 in the floor of the aqueduct of Sylvius immediately below that of the third nerve, 

 with which it is continuous. 



