SENSE OF SIGHT. 439 



in contnct with the aqueous humor, and is lined with a 

 prolongation of the membrane of Descemet (of the posterior 

 surface of the cornea, see Fig. 118, 13). Its posterior surface, 

 as we have said, is in close contact with the peripheral por- 

 tion of the anterior convexity of the crystalline lens, proving 

 that the so-called posterior chamber has no existence. The 

 periphery is continuous with the choroid tunic, to which this 

 diaphragm forms an appendage ; its central opening corre- 

 sponds with the centre of the crystalline lens, constituting 

 what is called the pupil of the eye. 



The structure of this membrane is similar to that of the 

 choroid : it contains a large number of vessels, pigment cells, 

 w r hich also form a thick layer upon its deep or posterior sur- 

 face (uvea), and of muscular fibres. The latter is the most 

 important element : it consists of fibres arranged in a circle 

 (sphincter of the pupil), and radiating fibres (dilatator 

 pupillw); 1 these fibres appear to be innervated by two dif- 

 ferent nerves, the sphincter or circular fibres by the motores 



1 Ch. Rouget's researches do not confirm the theory of the 

 existence of radiating or dilating fibres of the iris. This physi- 

 ologist has observed that in the iris of birds there are only muscular 

 fibres, arranged in a circle, and which may produce a contraction 

 of the pupil. He shows that the radiating bundles, which are con- 

 sidered as the dilating muscle of the pupil in the mammalia and 

 in man, really correspond with the veins of the iris when devoid 

 of blood. Therefore the iris is not in an active state when dilated, 

 as is the case when contraction of the pupil occurs : the latter 

 movement alone is active. A certain muscular arrangement will 

 sutlice to explain all the changes of the pupil, if the repose of the 

 iris be represented by the extreme state of dilatation. There is 

 great difficulty in observing this state of repose of the iris: the 

 pupil is seldom found to be entirely dilated, even after death; this 

 is due to the fact that the direct action of light (as shown by 

 Brown-Sequard), and the final contraction that produces cadaveric 

 rigidity in the muscles of animals after death, may each cause a 

 contraction of the pupil, which may last almost for an indefinite 

 period ; however, in the state of general relaxation of the muscular 

 system which follows prolonged inhalation of chloroform, a dilata- 

 tion of the pupil may be observed. Examination of the iris in 

 young inammiferous animals (cat and rabbit), a few days after 

 birth, before the eyelids a.re open, or before the organ of sight has 

 been excited by light, will show that the pupil is widely dilated, 

 and that the iris exists in the form of a narrow fillet: this appear- 

 ance is not owing to any want of development, for the induced 

 current of electricity will immediately produce as decided a con- 

 traction of the pupil as may be found in the adult. 



