880 OF SENSATION, AND THE ORGANS OP THE SENSES. 



is far greater for lines than for mere points ; since opaque threads of 14900th 

 of an inch in diameter (about half the diameter of the Silk-worm's fibre) may 

 be discerned with the naked eye, when held towards the light. The degree in 

 which the attention is directed to them has a great influence on the readiness 

 with which very minute objects can be perceived; and Ehrenberg remarks that 

 there is a much greater difference amongst individuals in this respect than 

 there is in regard to the absolute limits of vision. Many persons can distinctly 

 see such objects, when their situation is exactly pointed out to them, who 

 cannot otherwise distinguish them; and the same is the case with persons of 

 acuter perception, with respect to objects at distances greater than those at 

 which they can see most clearly. " I myself/' says Ehrenberg, "cannot see 

 l-2700th of an inch, black or white, at twelve inches' distance; but having 

 found it at from four or five inches' distance, I can remove it to twelve inches, 

 and still see the object plainly." Similar phenomena are well known in regard 

 to a balloon or a faint star in a clear sky, or a ship in the horizon : we easily 

 see them after they have been pointed out to us; but the faculty of rapidly 

 descrying depends on the habit of using the eyes in search of such objects, and 

 of attending to the sensory impressions received through them. 



882. The amount of light admitted to the Eye is regulated by the contraction 

 and dilatation of the Pupil, which is due to the muscularity of the Iris; its 

 smallest diameter being about l-20th, and its largest about l-3d of an inch. 

 The converging fibres of the iris are easily made out, as the membrane is prin- 

 cipally composed of them ; they have the general characters of the non-striated 

 -muscular fibre, but their nuclei are rounder and more loosely attached to the 

 contractile material. Although the principal direction taken by the fibres is 

 from the circumference towards the centre of the iris, yet their course is by no 

 means constantly straight, and they frequently anastomose with each other in 

 their passage; these anastomoses are most frequent near the pupil. The circu- 

 lar fibres of the iris are by no means so distinct ; and rarely, in Man, form 

 more than a somewhat undefined band, immediately surrounding the pupil, and 

 lying in front of the radiating fibres. Although the iris is so vascular, that 

 some anatomists have endeavored to explain its movements on the hypothesis 

 that they constitute a sort of erection, there is no ground for this idea ; and it 

 is more plainly demonstrated in many of the lower animals than it is in Man, 

 that the movements of the Iris are truly muscular, since we find the annular as 

 well as the radiating fibres very distinct, and in Birds (many of which seem to 

 possess a power of voluntarily regulating the diameter of the pupil) the former 

 are striated. The contraction of the annular fibres, whereby the diameter of 

 the pupil is diminished, is effected, as already explained ( 724), through the 

 instrumentality of the 3d Pair of nerves; the contraction of the radiating 

 fibres, on the other hand, whereby the pupil is dilated, is under the government 

 of the Sympathetic ( 847). The contraction of the Pupil takes place, as we 

 have seen, not merely for the purpose of excluding superfluous light from the 

 eye, but also that the most divergent rays may be cut off, when the object is 

 brought near the convex surface ( 878). 



[Prof. Julius Budge and Augustus Waller have discovered the very inte- 

 resting fact, that irritation of the cervical trunks of the sympathetic nerve, 

 by means of the magneto-electric machine, produces an extraordinary dilatation 

 of the pupil; and this alike in the rabbit, in which this trunk is isolated from 

 that of the pneumogastric, and in the cat and dog, in which the two trunks are 

 united. The phenomenon, they remark, is as constant as the contraction of 

 the leg when the sciatic nerve is galvanized. Thus is explained the permanent 

 contraction of the pupil after section of the sympathetic nerve in the neck, 

 which was first observed by Petit in 1712, but which, though subsequently 

 verified by other observers, has never led to more than a surmise that the cer- 



