ORGANS OF THE SENSES. 515 



The pigment cells constituting the posterior layer of the 

 iris, and called uvea, are irregular in size and form, as are 

 those also situated amongst the fibres of the iris ; upon the 

 varieties in the colouring matter contained in these last, many 

 of the differences observable in the iris of different persons 

 and animals depend. (See Plate LXYIIL^y. 14.) 



The muscular fibres of the iris in the human subject are 

 of the uristriped kind and follow two courses, a radiating and 

 a circular ; in birds, however, the radiating fibres consist of 

 striped muscular fibres, and they surround immediately the 

 pupil ; the one set of fibres dilates the pupil, the other con- 

 tracts it. 



The blood-vessels of the iris are very numerous, and are 

 derived chiefly from the two long ciliary arteries, which on 

 approaching the iris bifurcate and form an arch around it, 

 whence pass inwards a number of branches which form 

 loops near the pupillary margin. 



" On the anterior surface, near the pupil, a vascular circle 

 marks the line from which in the foetus the membrana pu- 

 pillaris stretched across in front of the pupil. This membrane 

 at that early period divides the posterior from the anterior 

 chamber, and receives from several parts of the circular vessel 

 last mentioned small branches, which approach the centre 

 and then return in arches after inosculating sparingly across 

 the central point." The membrana pupillaris is almost ab- 

 sorbed at birth. 



The iris, according to the authors of the Physiological 

 Anatomy, " is attached all round at the junction of the 

 schlerotic and the cornea, so near indeed to the latter that its 

 anterior surface becomes continuous in the following manner 

 with the posterior elastic lamina. This lamina, near its border, 

 begins to send off from its anterior surface, or that towards 

 the laminated cornea, a network of elastic fibres, which 

 stretch towards the border, becoming thicker as they ad- 

 vance, until at length the entire thickness of the lamina is 

 expended by being converted into them. These fibres then 

 bend backwards from the whole circumference of the cornea 

 to the circumference of the front of the iris, and are there 



