820 A MANUAL OF PHYSIOLOGY 



the mechanism of accommodation, and also fibres that innervate 

 the sphincter muscle of the iris, and thus cause contraction of the 

 pupil when light falls on the retina. Both groups of fibres ter- 

 minate by arborescing around sympathetic cells in the ciliary 

 ganglion, from which the path to the (unstriated) ciliary and 

 sphincter muscles is continued by post-ganglionic fibres. Further 

 back in the oculo-motor nucleus arise the motor fibres for four of the 

 extrinsic muscles of the eyeball and the elevator of the upper eyelid. 

 In the dog these fibres come off in the following order, from before 

 backwards : internal rectus, superior rectus, levator palpebrae 

 superioris, inferior rectus, inferior oblique. Most of the fibres of the 

 third nerve arise from nerve-cells on their own side of the middle line, 

 but a certain number decussate to enter the nerve of the opposite 

 side. Complete paralysis of the third nerve causes loss of the power 

 of accommodation of the corresponding eye, dilatation of the pupil 

 by the unopposed action of the sympathetic fibres, diminution of the 

 power of moving the eyeball, ptosis, or drooping of the upper lid, 

 external squint, and consequent diplopia, or double vision. 



The fourth or trochlear nerve arises from the posterior part of the 

 same tract of grey matter which gives origin to the third nerve. 

 It supplies the superior oblique muscle. Paralysis of the nerve causes 

 internal squint when an object below the horizontal plane is looked 

 at, owing to the unopposed action of the inferior rectus. There is 

 also diplopia on looking down. Unlike the other cranial nerves, the 

 two trochlear nerves decussate completely after they emerge from 

 their nuclei of origin. 



The fifth or trigeminus nerve appears on the surface of the pons 

 as a large sensory root and a smaller motor root. Its deep origin 

 is more extensive than that of any of the other cerebral nerves, 

 stretching as it does from the level of the anterior corpus quadri- 

 geminum above to the upper part of the spinal cord below. Its 

 sensory root, in fact, seems to include the sensory divisions of several 

 motor cranial nerves. 



The motor root arises partly from a nucleus (principal motor 

 nucleus] in the floor of the fourth ventricle below the pons, partly 

 from large round nerve-cells lying at the side of the grey matter 

 bounding the aqueduct of Sylvius all the way from the anterior 

 quadrigeminate body to the point at which the motor root is given 

 off (accessory or superior motor nucleus] . 



The fibres of the sensory root have their cells of origin in the Gas- 

 serian ganglion, whence they pass into the pons. Here they 

 bifurcate into ascending and descending branches. The ascending 

 branches end in the principal sensory nucleus, a collection of grey 

 matter at the side of the principal motor nucleus. The descending 

 branches, turning downwards into the medulla oblongata, terminate 

 in a long tract of scattered cells, constituting with the fibres the 

 so-called spinal root, and extending from the level of the second 

 cervical nerve through the medulla oblongata and the pons, where 

 it is continued into the principal sensory nucleus. The afferent 

 path is continued by the axons of cells of the sensory nuclei (or 

 nuclei of reception) of the nerve, many of which cross the middle 

 line and enter the intermediate fillet of the opposite side, and also 

 the special ascending bundle going to the thalamus. Some of the 

 axons do not decussate, but ascend in the fillet of the same side. 



The motor fibres of the fifth nerve supply the muscles of mastica- 

 tion and the tensor tympani. The sensory fibres confer common 



