796 



THE NERVOUS SYSTEM. 

 Fourth Nerve (Figs. 384, 479, 486). 



The Fourth or Trochlear Nerve (pathetic), the smallest of the cranial nerves, 

 supplies the Superior oblique muscle. Its apparent origin is behind the corpora 

 quadrigemina, from the valve of Vieussens, in the upper surface of which the 

 two nerves decussate. Its deep origin may be traced to the nucleus in the floor 

 of the aqueduct of Sylvius immediately below that of the third nerve, with which 

 it is continuous. After emergence from the surface of the valve of Vieussens, 



FIG. 480. Plan of the motor oculi nerve. (After Flower.) 



the nerve winds across the superior peduncle of the cerebellum and round the 

 crusta of the mid-brain, immediately above the pons Varolii, pierces the dura 

 mater in the free border of the tentorium cerebelli just behind, arid external 

 to, the posterior clinoid process, and passes forward in the outer wall of the cav- 

 ernous sinus, between the third nerve and the ophthalmic division of the fifth. 

 It then crosses the third nerve, and enters the orbit through the sphenoidal fissure. 

 It now lies at the inner extremity of the fissure internal to the frontal nerve. In 

 the orbit it passes inward above the origin of the Levator palpebrse, and finally 

 enters the orbital surface of the Superior oblique muscle. 



In the outer wall of the cavernous sinus this nerve receives some filaments 

 from the ophthalmic division of the fifth as well as from the cavernous plexus of 

 the sympathetic, and gives off a recurrent branch, which passes backward between 

 the layers of the tentorium, dividing into two or three filaments which may be 

 traced as far back as the Avail of the lateral sinus. In the sphenoidal fissure 

 it occasionally gives off a branch to assist in the formation of the lachrymal 

 nerve. 



Surgical Anatomy. The fourth nerve when paralyzed causes loss of function in the 

 Superior oblique, so that the patient is unable to turn his eye downward and outward. Should 

 the patient attempt to do this, the eye on the affected side is twisted inward, producing diplopia 

 or double vision. Accordingly, it is said that the first symptom of this disease which presents 

 itself is giddiness when going down hill or in descending stairs, owing to the double vision 

 induced by the patient looking at his steps while descending. 



Fifth Nerve. 



The Fifth or Trifacial Nerve (trigeminus) is the largest cranial nerve. It 

 resembles a spinal nerve (1) in arising by two roots ; (2) in having a ganglion 

 developed on its posterior root; and (3) in its function, since it is a compound 

 nerve. It is the great sensory nerve of the head and face and the motor nerve 

 of the muscles of mastication. Its upper two divisions are entirely sensory ; the 



