104 INNERVATION. [CHAP. XIX. 



The origin of the fourth nerve may be referred to the mesoce- 

 phale, from the superior surface of which it emerges in close con- 

 nexion with the testes, or immediately behind them. Its true 

 origin is doubtless from the olivary columns as they extend upwards 

 beneath the quadrigeminal tubercles. 



The fourth nerve emerges from the cranium through a canal in the 

 dura mater, situate near the posterior clinoid process of the sphenoid 

 bone, external to that in which the third nerve is lodged. It passes 

 along the outer wall of the cavernous sinus beneath the third nerve, 

 and in entering the orbit it rises above that nerve, and lies immedi- 

 ately beneath the periosteum, attaching itself to the orbital surface 

 of the superior oblique muscle, into the posterior third of which it 

 penetrates. 



This is the only muscle which the fourth nerve supplies, and the 

 nerve appears to be wholly lost in it. But as it passes through its 

 canal in the dura mater, it gives off a branch, which, taking a 

 retrograde course, passes into the tentorium cerebelli as far as the 

 lateral sinus, where it subdivides into two or three filaments. 



In its course it forms but few connexions, and those apparently 

 not constant. As it crosses the cavernous sinus it anastomoses with 

 the sympathetic, and, as it enters the orbit, with the lacrymal. 



That this nerve is the motor nerve to the superior oblique muscle 

 anatomy does not allow us to doubt, for the muscle receives no 

 other. 



Experiments and pathological observation have thrown no satis- 

 factory light upon its function. 



In animals of great power of expression, as in apes, according to 

 Sir C. Bell, this nerve, as well as the superior oblique muscle, is 

 large. 



Of the Sixth Pair of Nerves. These nerves emerge from between 

 the fibres of the anterior pyramids immediately behind the posterior 

 margin of the pons Varolii. It is not improbable that the true origin 

 of each nerve is from the central part of the medulla oblongata, the 

 olivary columns, and that the nerves pass between the fibres of the 

 pyramids without forming any real connexion with them. 



The sixth nerve has a straight, but very short intracranial course : 

 it penetrates the dura mater, and passes over the outside of the 

 carotid artery, between it and the lining membrane of the cavernous 

 sinus, entering the orbit between the two origins of the rectus ex- 

 ternus muscle, to the ocular surface of which it is entirely distri- 

 buted. 



As the sixth nerve is passing along the outer side of the internal 



