SECT. XV. PHYSIOLOGY. 119 
iris is the same; but there is little uniformity in 
this respect. 
ccLxxx. The iris is a flat circular ring, whose 
fibres converge like truncated radii, from the cir- 
cumference of a circle towards its centre; an 
opening, called the pupil, is thus left for the 
passage of the rays of light to the retina. Some 
authors have asserted the existence of circular 
fibres in the iris, which draw the central ends 
of the radiated fibres together, like a sphincter. 
Haller’s observation on this head, is made with 
all the candour of a philosophic mind: “ Verum 
non oportet fabricas excogitare, quas sensus non 
confirmant.” 
ccLxxxi. If the alternate expansion and con- 
traction of the iris depends on circular fibres, its 
irritability is quite an anomaly in the animal 
economy ; their existence, in fact, would imply 
that the optic nerve communicates to the fibres of 
the iris an expansive and contractile power at the 
same instant, which is incomprehensible. If the 
pupil were lessened by the contraction of circular 
fibres, it is evident that the central points of the 
radiated fibres would be drawn in a lateral direc- 
tion, whereas they move invariably in straight 
lines. Besides, in the lower animals, the iris is 
not always circular. When the iris of a cat is 
exposed to a strong light, the central ends of 
its inner and outer fibres arrange themselves in 
