THE CRANIAL NERVES 895 



optic tract. A lesion e.g., a tumour of the pituitary body involving 

 the whole of the optic nerve in front of the chiasma, would cause com- 

 plete blindness in the corresponding eye. Sometimes in disease of the 

 optic nerve vision is not totally destroyed in the eye to which it belongs, 

 but the field is narrowed by a circumference of blindness. In this case 

 the pathological change involves the circumferential fibres of the nerve. 

 When the chiasma is affected by disease, a very frequent symptom is 

 bitemporal hemianopia, blindness of the nasal halves of the retinae, with 

 loss of the outer or temporal half of each field of vision. The optic nerve 

 and tract contain a few efferent fibres for the retina, whose cell-bodies 

 have not yet been certainly located. 



The third nerve, or oculo-motor, arises from an elongated nucleus, 

 or a series of nuclei, containing large nerve-cells in the floor of the 

 Sylvian aqueduct below the anterior corpora quadrigemina. The root- 

 bundles coming off from the most anterior of the nuclei carry fibres that 

 innervate the ciliary muscle, and thus have to do with 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 terminate by arborescing around sympa- 

 thetic 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 palpebroe 

 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, ptosia, 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 quadrigeminum 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 Gasserian 

 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 



