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HANDBOOK OF PHYSIOLOGY. 



origin in front of the pons, at the median side of each crus. They de- 

 cussate with their fellows in the middle raphe. The nerve is connected 

 with the optic nerve. 4 



Function. It supplies the levator palpebrae superioris muscle, and 

 all of the muscles of the eyeball, except the superior oblique to which 



Fig. 364. Diagram of a longitudinal section through the pons, showing the relation of the 

 nuclei for the ocular muscles. CQ, corpora quadrigemina; 8. third nerve; in, its nucleus; 4, 

 fourth nerve ; iv, its nucleus, the posterior part of the third ; 0, sixth nerve. The probable 

 position of the centre and nerve fibres for accommodation is shown at a and o', for the reflex 

 action of iris, at 6, and b' ; for the external rectus muscles, at c, c'. The lines beneath the 

 floor of the fourth ventricle indicate fibres, which connect the nuclei. (Gowers.) 



the fourth nerve is appropriated, and the rectus externus which receives 

 the sixth nerve. Through the medium of the ophthalmic or lenticular 

 ganglion, of which it forms what is called the short root, it also supplies 

 motor filaments to the iris and ciliary muscle. The fibres which sub- 

 serve the three functions, accommodation, contraction of the pupil, and 

 nerve-supply to the external ocular muscles, arise from three distinct 

 groups of cells. 



When the third nerve is irritated within the skull, all those muscles 

 to which it is distributed are convulsed. When it is paralyzed or divided 

 the following effects ensue: (1) the upper eyelid can be no longer 

 raised by the levator palpebrse, but droops (ptosis) and remains gently 

 closed over the eye, under the unbalanced influence of the orbicularis 

 palpebrarum, which is supplied by the facial nerve : (2) the eye is turned 

 outward and downward (external strabismus) by the unbalanced action 

 of the rectus externus and superior oblique to which the sixth nerve is 

 appropriated ; and hence, from the irregularity of the axes of the eyes, 

 double sight, diplopia, is often experienced when a single object is within 

 view of both the eyes: (3) the eye cannot be moved either upward, down- 

 ward, or inward: (4) the pupil becomes dilated (mydriasis): (5) the eye 

 cannot accommodate for short distances. 



The IVth Nerve (Trochlearis) . 



Origin. The IVth nerve arises from a nucleus consisting of large 

 multipolar ganglion cells situated below, i.e., ventral to the aqueductus 

 of Sylvius, which extends from the back part of the nucleus of the 

 third nerve to the hind level of the posterior corpus quadrigeminum. 

 The fibres from either side sweep round the central gray matter, and 



