ANATOMY IN A NUTSHELL. 



391 



LESSON CX. (Plate CCXIII). 



The fourth cranial nerve or Pathetic or Trochlear, has its superficial 

 origin in the valve of Vieussens behind the corpora quadrigemina. The two 

 nerves decussate on the upper surface of the valve. Its deep origin is in the 

 floor of the aqueduct of Sylvius. This nerve can be seen on the base of the brain 

 on the outer side of the cms cerebri, just in front of the pons. It is the smallest 

 of all the cranial nerves but has the longest intercranial course. Coming from 

 the upper end of the valve of Vieussens this nerve crosses the superior peduncles 

 of the cerebellum and the crusta of the mid-brain, just beneath the pons, then 

 it pierces the dura mater in the free border of the tentorium, immediately be- 

 hind and external to the posterior clinoid process. From this point it passes 

 forward in the outer wall of the cavernous sinus between the third nerve and the 

 ophthalmic division of the fifth (See Plate CCXXXII) ; it then crosses the 



PLATE CCVI. 



TARSAL CARTILAGE 



The Muscles of the Orbital Region. 



third nerve and enters the orbit through the sphenoidal fissure (anterior Lacer- 

 ated); in this fissure it is the highest of all the nerves (See Plate CCXXIV) 

 being situated at the upper and inner angle, internal to the frontal nerve. In 

 the orbit it passes inward above the origin of the Levator palpebrae to enter the 

 orbital surface of the Superior oblique. In the cavernous sinus it communi- 

 cates with the ophthalmic division of the fifth, with the ciliary plexus, and semis 

 a recurrent branch backward between the layers of the tentorium as far as the 

 lateral sinus. While in the anterior lacerated fissure it may send a branch to 

 the lachrymal nerve. If the fourth nerve is paralyzed the function of the 

 Superior oblique is lost and one cannot turn the eye-ball now \\\ \ki> \\n out- 

 wai;i>. Should one attempl to do so the eye is twisted inward producing double 

 vision, diplopia. 



