SENSE OF VISION STRUCTURE AND ACTIONS OF IRIS. 759 



tending to the sensory impressions thus received (637). Aubert 1 found 

 that the excitability of the retina for extremely feeble stimuli rapidly at- 

 tained a high degree when a dark room was entered, though a slight in- 

 crease in sensitiveness continued to take place for a considerable period if 

 the sojourn in it were protracted. A wire heated by electricity was first 

 discovered when at a temperature of about 666 F. In another series of 

 experiments he found that the feeblest illumination of the field of vision 

 that could be recognized was about equal to that of the planet Venus when 

 brightest, or to the white light of day admitted into a chamber through an 

 opening forty-one seconds square. Small surfaces, however, always require 

 considerably greater illumination, in order that they should be perceived, 

 than large. 



618. The amount of light admitted to the eye is regulated by the con- 

 traction and dilatation of the Pupil, the smallest diameter of which is about 

 2 V n ' and its largest about -jd of an inch. The muscular structure of the 

 Human Iris is entirely of the non-striated kind, being composed of the elon- 

 gated fibre-cells, with staff-shaped nuclei which are characteristic of that 

 variety. Part of these are so disposed as to form a circular sphincter (Fig. 

 271, a), which can be readily seen in the iris of the white rabbit, or in the 

 blue iris of man from which the uvea has been removed, immediately sur- 

 rounding the pupillary margin to the breadth of about one-third of a line. 

 The fibres of this sphincter are not absolutely parallel, especially at the outer 

 margin, where they seem to become continuous with those of the radiating 

 fasciculi (b b), which may be traced from this sphincter (though usually with 

 difficulty) to the outer margin of the iris, sometimes anastomosing with each 

 other in their course. 2 The con- 

 traction of the annular fibres, FlG - 271 - 

 whereby the diameter of the pupil 

 is diminished, is effected, as already 

 explained ( 491), through the 

 instrumentality of the Third pair 

 of nerves ; the contraction of the 

 radiating fibres, on the other hand, 

 whereby the pupil is dilated, is 

 under the government of the cer- 

 vical portion of the Sympathetic, 

 being called forth (as MM. Budge 

 and Waller have shown 3 ) by irri- 

 tation of the trunk of the Sympa- 

 thetic in the neck, or of the lower 

 part of the cervical portion of the 

 Spinal Cord, by the magneto-elec- 

 tric apparatus ; whilst section of 

 the Sympathetic produces a per- 

 manent contraction of the pupil, 

 the action of the Third pair being 

 then no longer antagonized. It 

 appears, from other experiments, 



that the fibres through which this movement is effected, pass through the 

 Gasserian ganglion, and are distributed to the eye by the ophthalmic branch 



1 Physiologic der Netzhaut, 1864. 



2 See Prof. Kolliker's Mikroskopische Anatomic, 3d. ii, \ 272; and Joseph J. Lister's 

 Observations on the Contractile Tissue of the Iris, in Quart. Journ. of Microscop. 

 Science, vol. i, p. 8. See Lancet, vol. i, 1870, pp. 127 and 211. 



3 Gazette Medicale, 1851, Nos. 41, 44. 



Muscular structure of the Tris of a White Rabbit: 

 n, sphincter of the pupil ; b, b, radiating fasciculi of 

 dilator muscle; c, c, connective tissue with its cor- 

 puscles. 



