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VETERINARY PHYSIOLOGY 



eye. At the same time the centre for the ciliary muscle 

 is also called into play to produce accommodation. 



The centre for dilatation of the pupil is situated in the 

 medulla oblongata. Like the centre of the sphincter it may 

 be reflexly excited, stimulation of ingoing nerves causing a 

 dilatation of the pupil when the medulla is intact (fig. 58). 



The dilator fibres pass down the lateral columns of the 

 spinal cord to the lower cervical and upper dorsal region, where 

 they arborise round cells in the anterior horn. From these, 

 fibres pass by the anterior root of the second (2 D. N.\ possibly 



FIG. 58. Nerve Supply of the Intrinsic Muscles of the Eye (see text). 



also of the first and third dorsal nerves, and, passing up 

 through the inferior cervical ganglion, run on to the superior 

 ganglion, where they arborise round cells which send axons 

 to the G-asserian ganglion of the fifth cranial nerve (V.), and 

 from there the fibres pass along the ophthalmic division and its 

 long ciliary branches to the dilator fibres of the iris (D.P. ). 



The importance of the course taken by these dilator fibres 

 is considerable, because diseases of the spinal cord in the lower 

 cervical and upper dorsal region (the eilio-spinal region), and 

 tumours in the upper mediastinum, may interfere with their 

 action, and by stimulating cause chronic dilatation of the pupil, 

 or by paralysing prevent dilatation of the pupil. Since the 



