NERVE 149 



a dilatation of the pupil when the medulla is intact 

 (fig. 69). 



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 (cilio- 

 spinal region). From these, fibres pass by the anterior root 

 of the second (Fig. 69, £? D.xY.), 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 cranial nerve (V.), which 

 course through this and pass along the ophthalmic division 

 and its long ciliary branches to the dilator fibres of the 

 iris (D.P.). 



There is evidence of the existence of a peripheral 

 mechanism in the iris. The pupil may be seen to contract 

 and dilate in the eye of a cat after decapitation, and various 

 drugs act directly upon it. Atropin causes a dilatation and 

 physostigmin and pilocarpin cause a contraction. Adrenalin 

 causes dilatation when placed in the eye of a mammal, but 

 only after removal of the superior cervical ganglion of the 

 same side, which seems to render the peripheral mechanism 

 more sensitive. A nerve plexus exists in the iris, and this 

 probably acts upon the muscular fibres. 



3. Range of Accommodation. — The power of positive accom- 

 modation varies at different ages, being greatest in young 

 animals, because in early life the lens is most convex and 

 more elastic. 



Presbyopia. — The " range of accommodation," i.e. the 

 difference between the " near point " and the " far point," 

 steadily decreases as age advances till the condition of 

 Presbyopia — old-sigh tedness — is produced. 



Imperfections of the Dioptric Mechanism. 



(1) Hypermetropia. — The eye may be too short from before 

 backwards, and thus, in the resting state, parallel rays are 

 focussed behind the retina, and, in order to see even a distant 



