26 INNERVATION. [CHAP. XVII. 



The varieties of colour in the eyes of different animals and indi- 

 viduals depend almost solely on the colour of the front of the iris, 

 which itself resides chiefly in pigment- cells, situated in its substance 

 rather than as a layer on its anterior surface. These cells are most 

 irregular in shape and size, and lie in the interstices of the more 

 essential tissues, which they much obscure. The iris is consequently 

 best examined in albino specimens. 



The iris is undoubtedly contractile, and the anatomical charac- 

 ters of its principal tissue so nearly resemble those of unstriped 

 muscle, that it may be considered as a variety of that tissue. Its 

 fibres are loaded with nuclei which are rather rounder than those of 

 unstriped muscle, and more loosely attached to the contractile ma- 

 terial. The principal direction taken by the fibres is towards the 

 pupil, although they are more or less meandering and interlacing 

 in this course. Arrived near the pupil, they appear to join, and 

 form indistinct arches. In many instances it is easy to detect a set 

 of circular fibres, either gathered into a principal bundle near the 

 pupil or more diffused, but always lying in front of the others. 

 These seem to answer to the circular fibres of the bird's iris, 

 which are of the striped variety, and occupy the front of the 

 membrane. There may also be usually distinguished in the very 

 thin margin of the pupil an arrangement of fibres more circular 

 than radiating. 



The iris is so vascular that some anatomists have considered it 

 erectile, and have erroneously ascribed its movements to this pro- 

 perty. But its vessels are slender and delicate, and resemble those 

 of the unstriped muscle. They are derived chiefly from the two 

 long ciliary arteries, which on approaching it bifurcate, and form a 

 circle around it, whence pass inwards a great number of minute 

 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 pupillaris stretched 

 across in front of the pupil. This membrane at that early period 

 divides the anterior from the posterior chamber, and receives from 

 several parts of the circular vessel last mentioned, small branches 

 which approach the centre, and then return in arches, after inoscu- 

 lating sparingly across the central point. The membrana pupillaris 

 is almost absorbed at birth. 



The grayish structure coating the choroid for about an eighth of 

 an inch behind the cornea, presents at its anterior edge a more 

 white and opaque circle, the ciliary ligament, which seems to be 

 chiefly of a fibrous character, and to connect the border of the 



