;io Nov., 1908.] Elements of Animal Physiology. 68] 



Vision, it may be stated, presents a number of curious features such as 

 .contrast -phenomena, after images, &c. in fact no sense organ can be so 

 -easily tricked as the eye. One important facultx which has a distinct 

 significance is the power of the retina to adapt itself to feeble illuminations, 

 being able, after a certain interval, to see with fair distinctiveness, objects 

 which were at first hidden in darkness. Behind the retina in most domestic 

 animals is a glistening coat, the tapetum, which reflects light and gives 

 to the eyes of these animals a peculiar glare in low^ illuminations. 

 The eyes of all mammals present certain optical defects. 



1. The .system is not properly centred, that is, the centres of the cornea, 

 the pupil, the lens and the retina do not lie in a straight line. 



2. The curvature of the cornea, as also that of the lens, is not regular. 

 Such a departure from the purely spherical means a distortion of the 

 image on the retina. This defect is present in all eyes and is very marked 

 in those of the lower animals whose vision must be far from perfect. To 

 it the name astigmatism has been given. 



3. A very common failing is myo-pia, or short-sightedness, due to the 

 fact that the distance from the lens to the retina is too great and in conse- 



'Quence the image of a far object is focussed in front of the retina. This 

 defect can be partly remedied in the human being by wearing concave 

 glasses. It is very common in the lower animals and is often associated 

 with marked astigmatism. 



4. In hypermetropia the lens is too near the retina and objects at a dis- 

 tance require some focussing effort, whilst near objects cannot be seen 

 clearly at all or only so by a violent action of the ciliary muscle. This is 

 remedied in the human eye by wearing convex lenses. 



The eyeball can move in its socket to a limited extent. Four small 

 muscles, called recti, are attached to the outer surface of the sclerotic and, 

 by their contraction, can turn the eyeball so that the cornea looks up, or 

 down, or backwards, or forwards (outwards or inwards in man), or a com- 

 bination of two of these. The eyeball can be rotated to a slight degree in 

 a direction with or against the hands of a watch by two other small muscles 

 called the superior and inferior oblique; by this movement the pupil can 

 remain horizontal in whatever position the head is. Slight protrusion and 

 retraction of the eveball from the socket can occur in mo.?t animals through 

 special muscles innervated by the thoracic autonomic. 



II. THE EA"R. — The ear in many respects is a mechanism superior to 

 the eye. Owing to the qualities of the air-waves to which it responds the 

 ear need not be directed towards the source of the sound. INIoreover it has 

 a greater analytic power, being able to distinguish two notes when sounding 

 together, and has the faculty of recognising noises as well as musical tones. 

 The external ear has some slight action in collecting the sound waves and in 

 partially shielding the ear from all sounds except those to which the atten- 

 tion is directed. The sound waves enter the curved canal called the 

 EXTERNAL AUDITORY MEATUS and then Strike against a thin membrane, called 

 the EAR DRUM, which completely separates the meatus from a chamber 

 within called the tympanic cavity. The ear drum is pet in movement by 

 the. sound waves in much the same manner as the disc in a telephone 

 receiver. To the inner or tympanic aspect of the drum is attached one end 

 of a bridge consisting of three small bones (ossicles) which stretches across 

 the tympanic cavitv and conveys the vibrations of the drum tothe wall on 

 the opposite side. "The tympanic cavitv contains air which is in communi- 

 cation with the air in the upper part of the pharynx by means of a special 

 air-pipe— the Eustachian tube. This tube, however, opens only in the act 



