674 STRUCTURE OF THE BULB, PONS, AND MID-BRAIN [CH. XLYI. 



the grey matter on the side of the Sylvian aqueduct underneath the 

 superior corpus quadrigeminum, and close to the middle line. The 

 anterior part of this nucleus is composed of small cells from which 

 small nerve-fibres originate for the ciliary muscle and sphincter of 

 the iris (intrinsic muscles of the eyeball). These fibres correspond 

 to the visceral fibres of a spinal nerve, and, like them, have a cell 

 station, namely, in the ciliary ganglion. The posterior part of the 

 nucleus is composed of larger cells, and these give rise to larger 

 fibres which supply the following extrinsic eye-muscles: superior 

 rectus, inferior rectus, internal rectus, inferior oblique and levator 

 palpebrse. 



The fourth nerve (trochlear) takes origin from the grey matter 

 immediately below the centre of the third, but slightly more lateral 

 in position. It is underneath the inferior corpus quadrigeminum. 

 It supplies the superior oblique muscle of the opposite eyeball. 



The sixth nerve (abducens) arises from a centre beneath the 

 eminentia teres in the upper part of the floor of the fourth ventricle 

 near the middle line. It supplies the external rectus. 



It is obviously necessary that the eye-muscles should work 

 together harmoniously, that the two eyeballs should also be moved 

 simultaneously and in corresponding directions, and that such move- 

 ments should take place in accordance with the necessities of vision. 

 This is provided for in the shape of association fibres which link the 

 centres of the eye-muscles together. The principal association tracts 

 are the posterior longitudinal bundle, which connects the nuclei of 

 the third and sixth nerves, and the ventral longitudinal bundle 

 which unites the optic nerves through the intermediation of the cells 

 of the C. quadrigemina, with the nuclei of all these nerves. It should 

 also be remembered that all the fibres of the fourth, and some of 

 those of the third nerve, decussate in the middle line. 



The fifth nerve (trigeminal) is a mixed nerve ; it leaves the side of 

 the pons in a smaller motor, and a larger sensory division. The 

 former supplies the muscles of mastication, the tensors of the palate 

 and tympanum, the mylo-hyoid, and the anterior belly of the 

 digastric; the sensory division has upon it a ganglion called the 

 Gasserian ganglion; it is the great sensory nerve of the face and 

 head. The motor fibres arise from the motor nucleus (Vra, fig. 408), 

 which lies at the lateral edge of the upper part of the floor of the 

 fourth ventricle, but a certain number of its fibres arise from cells 

 in the lower part of the mid-brain and upper part of the pons; 

 this long stretch of nerve-cells, indicated by the long blue tail in the 

 diagram, is called the accessory or superior motor nucleus of the fifth. 

 The sensory fibres arise from the cells of the Gasserian ganglion, 

 which resemble in structure those of a spinal ganglion ; one branch 

 of each passes to the periphery in the skin of the head and face, and 



