760 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



of the Fifth pair ( 490). The contraction of the Pupil is a reflex action, 

 induced by excitation of the retina, and acting through the nervous circle 

 formed by the optic nerves and tracts, and the third pair of cerebral nerves. 

 It answers the purpose, as we have seen, not merely of excluding superfluous 

 light from the eye, but also of cutting oft' the most divergent rays when the 

 object is brought near the refracting surface ( 612), and of thus preventing 

 the indistinctness of vision which would result from their admission. For, 

 although, when the Eye is at rest, parallel rays come to a focus on the Retina, 

 divergent or convergent rays proceeding from any point come to a focus 

 either behind or in front of that membrane, forming upon it what is termed 

 a "circle of dispersion ;" but by cutting off the outermost rays and permit- 

 ting the entrance of those only which are nearly parallel, the magnitude of 

 such circles is materially diminished, and vision correspondingly improved. 

 It is interesting to observe that contraction of the pupil is effected with 

 greater rapidity than dilatation, a matter of some importance in sudden 

 alternations from light to darkness. Contraction of the pupil may also be 

 produced by powerfully converging the optic axes, as iu squinting inwards; 

 by strong accommodation of the eyes for near objects, and, as already stated, 

 by certain poisons, as the Calabar bean, uicotiu, opium, etc. M. Brown- 

 Sequard 1 has shown that the Irides of various animals may be directly affected 

 by Heat and Light. Thus if the eyes of dogs, cats, or rabbits, either soon 

 after death, or shortly after their removal from the body, be suddenly ex- 

 posed to an alteration of temperature amounting to 50 or 60 F., if the 

 pupil be contracted, it expands, though on the contrary, if it be in the first 

 instance dilated, it speedily contracts. These effects of heat have not been 

 observed either in Man or the Mammalia during life. As regards the direct 

 action of Light, Brown -Sequard found that the Irides of eels and frogs 

 which had been removed for several (even sixteen) days from the body in 

 winter; as well as the Irides of mammals and birds for a short time after 

 death, contracted on exposure to the light of the sun or of a caudle; this 

 effect being produced even when the posterior half of the eye had been cut 

 off, and when, therefore, there could have been no reflex action. The yel- 

 low rays appeared to have the greatest energy. These remarkable phenom- 

 ena prove, at all events, that the contraction of the pupil is not, as has 

 occasionally been maintained, exclusively due to vascular turgescence. The 

 cause of the effect produced by Light is difficult to explain, but that occa- 

 sioned by Heat may perhaps be attributed, as Brown-Sequard suggests, to 

 the circumstance that a contracted muscle has less power than one in a state 

 of extension, so that if a stimulus act equally on both, when previously nicely 

 balanced in point of strength, the uncontracted muscle will exert the greater 

 force, and consequently overpower the contracted muscle. Dilatation of the 

 pupil may be caused by strong irritation of sensory nerves, and occurs dur- 

 ing violent muscular effort, and in dyspnoea, as well as from the action of 

 belladonna. 



619. The sense of Vision depends, in the first place, on the excitement of 

 our sensational consciousness by the ocular picture impressed upon the ret- 

 ina, which represents the outlines, lights and shades, colors, and relative 

 positions of the objects before us; and all the ideas respecting the real forms, 

 distances, etc., of bodies, which we found upon these data, are derived through 

 the perceptions, either instinctively or experieutially suggested by sensations. 

 Many of these ideas are derived through the combination, in our minds, of 

 the Visual perceptions, with those derived from the sense of Touch. Thus, 

 to take a most simple illustration, the idea of smoothness is one essentially 



1 Journal dc In Physiologic, 1859, vol. ii, p. 294. 





