438 ORGANS OF THE SENSES. 



increases, especially in the central part of its anterior surface, 

 this centre being the only part that is free and susceptible of 

 deformation, because the iris prevents any thing of the kind 

 from taking place in the periphery. The space which was 

 supposed to exist between the iris and the crystalline lens, 

 and which was called the posterior chamber, has really no 

 existence, the iris coming in immediate contact with the 

 entire corresponding surface of the crystalline lens (Rouget). 

 The contractions of the iris may, perhaps, also affect the 

 shape of the lens ; at all events, the iris appears to lend an 

 important aid to sight, since we see persons who, although 

 they possess to perfection the faculty of accommodation, are 

 yet unable to see clearly, because they have lost the power 

 of contracting the iris, which has been destroyed or injured. 



Ch. Rouget has shown, in describing the internal or an- 

 nular ciliary muscle, that this muscle, in contracting, com- 

 presses the irido-choroid venous trunks, forces all the blood to 

 pass through the ciliary process, and thus gives rise to the 

 erection or rigidity of these organs ; without these phenom- 

 ena the ciliary muscles would have no action on the crys- 

 talline lens. None of the theories of accommodation were 

 able, by the help of known facts, to explain the direct influ- 

 ence upon the crystalline lens : this effect is produced by the 

 annular ciliary muscle ; the first contractions of this muscle 

 obstruct the flow of blood through the veins, and cause the 

 erection of the ciliary process ; and, while in this state, these 

 organs become fitted to transmit to the crystalline lens, in a 

 modified form, the pressure exercised by the ciliary muscle. 



We see, in short, that the contractions of the anterior 

 portions of the choroid coat (ciliary muscle) have the effect 

 of producing accommodation. This adaptation is involuntary 

 and spontaneous, and is the consequence of a reflex phenom- 

 enon : it seems as if the retina or the central organs of sight, 

 perceiving the confusion of the image produced, reacted upon 

 the ciliary muscles, thereby causing their contraction. The 

 ciliary or ophthalmic ganglion was long considered as the 

 centre of these reflex phenomena, but they are now supposed 

 rather to belong to the cephalic or cerebral portion of the 

 cord (Pons Varolii quadrigemina and corpora, see p. 59). 

 The muscular fibres of the choroid tunic are smooth: this 

 accounts for a certain slowness in the accomplishment of the 

 process of accommodation. 



B. The iris is really a diaphragm situated in the camera 

 obscura formed by the eyeball: its anterior surface is 



