16 STRUCTURK OF THE EYE. 



and division of this nerve is followed by dilatation. Irritation of the supe- 

 rior cervical ganglion, on the other hand, induces dilatation, while its 

 destruction or removal is succeeded by contraction of the pupil. More- 

 over, after removal of this ganglion on one side, the application of bella- 

 donna, or administration of strychnine, is no longer followed by any 

 marked dilatation of the pupil of the corresponding eye, but that of the 

 opposite eye becomes extremely dilated. Besides thus proving the double 

 source of nervous influence supplied to the iris, these experiments appear 

 also to establish the truth of the opinion that dilatation of the pupil is as 

 much the result of an active state of the iris as is its contraction, and that 

 the one act is most probably produced by the radiating fibres of this struc- 

 ture, the other by the circular fibres situated around its inner margin. 



The real nature of the fibres of which the iris is principally composed 

 still remains obscure, although little doubt can exist that muscular tissue 

 constitutes some portion of this membrane. Indeed, independent of the 

 proof afforded by the rapidity of the iridal movements, and of their ready 

 excitation on stimulating the nerves by which the iris is supplied, M. 

 Briicke* states that the existence of a large quantity of fibres precisely 

 analogous to those of which the muscular walls of the intestines are com- 

 posed may be observed in the iris of the human eye, mixed up with the 

 bundles of connective tissue which is frequently described as constituting 

 the sole fibrous texture of this membrane. The existence of these mus- 

 cular fibres in the iris of many animals, such as ruminants, appears to be 

 doubtful. It is well known that in the iris of birds distinct muscular 

 fibres possessed of transverse striae are found in abundance ; and that in 

 accordance with this anatomical fact a voluntary power is possessed over 

 the movements of this membrane. 



The contraction of the pupil which ensues on irritation of the third 

 nerve after death is never so complete as that observed during life, whereas 

 the dilatation resulting from irritation of the superior cervical ganglion is 

 as complete as during life. These facts are considered by M. Guarini to 

 prove that some share in the contraction of the pupil during life should be 

 attributed to venous congestion, resulting from compression of the blood- 

 vessels interlaced among its fibres by the circular portion of the iris during 

 its contraction. This is, probably, the utmost that can be effected by 

 vascular turgescence towards the production of the iridal movements, 

 although by many physiologists the contraction of the pupil has been 

 regarded as almost entirely due to such turgescence, and its dilatation to 

 an empty state of the vessels. 



Irritation of the third pair of nerves does not appear in all cases to be 

 followed by contraction of the pupil. Volkmann | found, for example , 

 that although in dogs, after cutting off the head, and removing the brain, 



* Miiller'8 Archiv. 1846, p. 377. t Muller's Archiv. 1845, p. 414. 



