THE BRAIN. 201 



4. The Trochlear. Figs. 21, 29, 34, 10, n. 



This is found curving forward around the outer side of 

 the crura cerebri. It enters the cavernous sinus and leaves 

 the skull through the sphenoidal fissure. See page 50. 

 (The deep origin of the nerve is from a nucleus in the floor 

 of the aqueduct of Sylvius behind the third nucleus. The 

 fibres of the fourth nerve decussate across the valve of 

 Vieussens, then proceed on their way around the crura 

 cerebri.) 



5. The Trig-emiims. Figs. 21, 29, 53, 10, n, 12. 



The superficial origin of this nerve is from the side of 

 the pons in two bundles, a larger anterior (sensory) and a 

 smaller posterior (motor). These bundles pass forward 

 over the inner end of the petrous portion of the temporal 

 bone, and upon the front surface of the petrous bone the 

 sensory root terminates in the Gasserian ganglion while the 

 motor root passes beneath the ganglion to the foramen 

 ovale. See page 46.- 



(The deep origin of the fifth nerve is from three sets of 

 nuclei ; one in the floor of the aqueduct of Sylvius, from 

 which comes the descending 1 root ; a middle from the floor 

 of the fourth ventricle, from which arises the middle root ; 

 and a third nucleus from the upper part of the cervical 

 division of the spinal cord, from which arises the ascending- 

 root. The descending root is sensory and trophic, the 

 middle motor and sensory, and the ascending sensory.) 



Ophthalmic division see page 54. Superior maxillary divi- 

 sion see page 149. Inferior maxillary division see page 117. 



6. The Abducens. Figs. 29, 10, n. 



The superficial origin is in the groove between the pons 

 and medulla, and from between the olivary body and the 

 anterior pyramids of the latter. The nerve continues for- 



