[Verse: "If the whole body were an eye, where were hearing? if the whole were hearing, where were smelling?"--CORINTHIANS XII.] 978. _Why do we feel inconvenienced by sudden light?_ Because an excess of light enters the eye before the _iris_ has had _time to adjust the pupil_ to the amount of light to be received. 979. _Why if we look upon a very bright light, and then turn away, are we unable to see?_ Because the _iris_ has so reduced the pupil while we were looking at the bright light, that immediately upon turning to a darker object, _the pupil is too small_ to admit sufficient rays to enable us to see. [Illustration: Fig. 64.--CAPILLARY BLOOD-VESSELS OF THE EYE.] A A. Capillary veins distributed over the _sclerotic coat_. B. One of the trunks of the _optic nerve_. C. A _nerve_ communicating with the _ciliary processes_. D. A _vein_ running parallel with the nerve to the _ciliary processes_. E. Side view of the _iris_. 980. _Why do we see better after a short time?_ Because the _iris_ has relaxed and enlarged the pupil, therefore _we receive more rays of light_ from the comparatively dark object, and are enabled to see it more clearly. 981. _Why do cats, bats, owls, &c., see in the dark?_ Because their eyes are made highly sensitive to _small quantities of light_. It is also believed that there are certain properties of light which affect their eyes, but do not affect ours. In other words, that there are some rays which are luminous to them which are not luminous to us. Hence they find _light_ in what we call _darkness_. [Verse: "He that hath ears to hear, let him hear."--MATTHEW XI.] 982. _Why does the pupil of a cat's eye appear nearly closed by day?_ Because the cat's eye is so sensitive to light that the iris _closes the pupil almost entirely_ to shut out the too powerful light. CHAPTER XLIX. 983. _Why do we hear?_ Because the _tympanum_ of the ear _receives impressions from sounds_, and transmits those impressions to the brain in a similar manner to that in which the retina of the eye transmits the impressions made upon it by light. 984. _Why is one part of the ear spread out externally?_ The external ear is a _natural ear-trumpet_, and serves to collect the vibrations of sound, and to conduct them towards the internal ear. 985. _Why is the ear allowed to project, whilst the eye is carefully enclosed?_ Because the external ear, being formed of tough cartilaginous substance, and being very simple in its organisation, is but little liable to injury. 986. _Why do hairs grow across the entrance of the ears?_ Because they prevent the intrusion of insects, and of particles of dust, by which otherwise the faculty of hearing would be impaired.