THE SENSES 



D.f- 



S.CG 



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. 50). 



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 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 Gasserian 

 ganglion of 

 the fifth cran- 

 ial nerve (F.), 

 and from there 



the fibres pass along the ophthalmic 

 division and its long ciliary branches to 

 the dilator fibres (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 cilio-spinal region), and 

 tumours in the upper mediastinum, may 

 interfere with their acjj^, and by stimu- 

 lating cause 

 pupil, or by 

 of the pupi 

 fibres of th 

 animals, it h) 



FIG. 50. Nerve Supply 

 of the Intrinsic Muscles 



^^^^^^ ^^^ of the Eye (see text). 



e dilaB muscle 



fff been demonstrated in all 

 ^gested that the nerve may act by 

 inhibiting the sphincter pupilloe, but the evidence on this 

 point is not conclusive. 



The non-striped muscle of the iris, like non-striped muscle 

 elsewhere, may act independently of nerve fibres as may be 

 seen in the eye of a decapitated cat. Further, various drugs 

 seem to act directly upon them e.g., physostigmin causes a 

 contraction, while atropin causes a dilatation. 



8 



ID.N. 

 2D.N 



Bptation of the 

 wejBydilata\ion 



