VISION. 23l 



of the iris, tlie blood-vessels of the choroid form a corona of 

 richly vascular projections directed inwards, and capable of 

 considerable variation in size according to their degree of 

 gorgement: these are called the ciliary processes; and con- 

 nected with their roots is the ring of tissue which fastens the 

 iris to the sclerotic, a white ring consisting principally of 

 muscular fibres, the ciliary muscle. 



The whole of the deep surface of the choroid and back of 

 the iris is lined with epithelial cells loaded with dark brown 

 pigment, which opposite the choroid are flat, hexagonal, and 

 arranged in a single layer, but behind the iris are more 

 densely accumulated. In persons with brown eyes, the 

 colour of the iris is due to a deposit of pigment between its 

 anterior fibres, corresponding with the branched pigment- 

 corpuscles between the vessels of the choroid; but in those 

 with blue eyes, the iris is devoid of pigment, and the colour 

 is due to the effect of the dark pigment behind its white 

 substance. In Albinos, persons in whom there is an absence 

 of brown pigment from all the situations in which it is com- 

 monly found, the white hair and eye-lashes are accompanied 

 with pink eyes, the colour of which is due to the blood, 

 lighted up, not only with reflected, but also transmitted light. 



The pigment of the choroid prevents reflection of rays from 

 the parts of the interior of the eye on which they fall, to other 

 points of the sensitive surface, which, were it to occur, would 

 blur the image of the landscape, as is illustrated by the 

 imperfect vision from which albinos suffer. But, if the 

 student choose for dissection the eye of a sheep or some 

 other domestic animal, he will find on the inner surface of 

 the choroid a state of matters not existing in man, namely, a 

 portion, opposite and above the pupilary aperture, which has 

 an intense satin-like whiteness. This is called the tapetum, 

 and by reflecting the rays to the part of the sensitive surface 

 immediately in contact with it, is of service in utilising a dim 

 light, and enabling these animals to see at night. It is this 

 tapetum seen through the humours, which gives the brilliant 

 greenish light to the pupil of the cat. 



171. In the course of dissection, let the choroid coat be gently 

 torn open and raised from the subjacent structures. If this 

 be done with due care, there will be seen laid over a globe of 



