THE THIRD NERVE. 



229 



The fibres of the nerve spring mainly from the oculomotor nucleus in the grey 

 matter of the floor of the aqueduct of Sylvius opposite the superior quadrigeminal 



N.HI 



Fig. 146. PLAN OF THE ORIGINS OP THE THIRD, FOURTH AND 

 SIXTH NERVES. (Modified from Gowers. ) 



The nerves and their nuclei are projected into the outline of 

 a median section of the mid brain, and pons : III, third nerve ; 

 N.III, its nucleus; IV, fourth nerve; N.IV, its nucleus; 

 P.L. B., posterior longitudinal bundle; VI, sixth nerve; N.VI, 



its nucleus. 



body (figs. 146 and 151) ; to these are added others 



which ascend in the posterior longitudinal bundle 



from the nucleus of the sixth nerve of the opposite 



side. They pass ventrally through the tegmentum, 



and emerge in ten to fifteen bundles from the inner 



side of the crus cerebri, commencing close to the 



upper border of the pons, and extending upwards 



and outwards along the line of the oculomotor 



groove. One of these bundles is frequently separated 



from the rest, and issues more externally from the anterior surface of the 



crus. 



internal 

 carotid artcry'ty 



posterior 

 communicating- 

 artery 



posterior 



cerebral 



artery 



inferior 



longitudina^ 



sinus 



optic nerve 



fc third nerve 



tjjs' --fourth nerve 



superior cerebellar 

 artery 



/_' tentorium 



inferior 



quadrigeminal 



body 



V *2&7 



- -\- falx cerebri 



Fig. 147. THE THIRD AND FOURTH NERVES IN THEIR INTRACRANIAL COURSE. 



(Drawn by T. W. P. Lawrence.) 



The mid-brain is divided in the aperture of the tentorium, and the cerebrum removed. On the 

 right side the posterior cerebral and posterior communicating arteries are cut short in order to expose 

 the origin of the third nerve. On the left side the tentorium and crus cerebri are slightly separated so 

 as to show the fourth nerve more fully. 



