CH. XLIV.] THE FIFTH AND SEVENTH NERVES 641 



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. 459), 

 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 

 the other grows centralwards ; on reaching the pons these bifurcate, 

 the ascending branches arborise around the principal sensory nucleus 

 of the fifth (Vd, fig. 459), which lies just lateral to the motor nucleus, 

 while the descending branches pass down into the bulb, where they 

 form the descending root of the fifth, and some reach as far down 

 in the spinal cord as the second cervical nerve. Mingled with these 

 descending fibres are numerous nerve-cells, many of which are grouped 

 in clusters (islands of Calleja), and the descending fibres form synapses 

 around them. The new axons arising from the cells of the sensory 

 nuclei pass upwards in three principal tracts: (1) The greater 

 number cross the raphe and join the mesial fillet ; (2) some ascend 

 the fillet of the same side ; and (3) others pass into a special ascending 

 bundle which lies near the ventricular floor (the central tract of the 

 cranial sensory nerves). 



The seventh nerve (facial) is the great motor nerve of the face 

 muscles. It also supplies the platysma, the stapedius, stylo-hyoid, 

 and posterior belly of the digastric. When it is paralysed, the 

 muscles of the face being all powerless, the countenance acquires on 



2 S 



