t>8 SPECIAL SENSES. 



which is designated the HOROPTER, a circle (fig. 43) which 

 passes at once through the point of coincidence, /, of the visual 

 axes, I a, I b, and the points of decussation, c c\ of these axes 

 with the lines of direction.]* 



135. The eye constructed as above described, is called a 

 simple eye, and belongs more especially to the vertebrate ani- 

 mals. In man, it arrives at its highest perfection. In him, 

 the eye also performs a more exalted office than mere vision. 

 It is a mirror in which the inner man is reflected. His pas- 

 sions, his joys, and sorrows, are reflected with the utmost 

 fidelity, in the expression of his eye, and hence it has been 

 called " the window of the soul." 



136. Many of the invertebrate animals have the eye con- 

 structed upon the same plan as that of the vertebrate animals ; 

 the optic nerves, which form the retinae, are derived from the 

 cephalic ganglia, a nervous centre analogous to the brain. 

 The eye of the cuttle-fish contains all the parts essential to 

 that organ in the superior animals, and, what is no less im- 

 portant, the eyes are only two in number, and placed upon the 

 sides of the head. 



137. The snail and kindred animals have, in like manner, 

 only two eyes, mounted on the tip of a long stalk (the ten- 

 tacle}, or situated at its base, or on a short pedicle by its side. 

 Their structure is less perfect than in the cuttle-fish, but still 

 there is a crystalline lens, and more or less distinct traces of 

 the vitreous body. Some bivalved mollusca, the pectens for 

 example, have a crystalline lens, but instead of two eyes, they 

 are furnished with numerous eye spots, which are arranged 

 like a border around the lower margin of the animal. 



138. In spiders, the eyes are likewise simple, and usually 

 eight in number. These little organs, called ocelli, instead of 

 being placed on the sides of the body or of the head, occupy 

 the anterior part of the cephalo- thorax. All the essential parts 

 of a simple eye, the cornea, the crystalline lens, the vitreous 

 body, are found in them, and even the choroid, which presents 

 itself in the form of a black ring around the crystalline lens. 

 Many insects, in their caterpillar state, have also simple eyes. 



139. Rudiments of eyes have likewise been observed in 

 many worms. They generally appear as small black spots 

 on the head ; such as are seen on the head of the leech, the 



* Professor Wagner's Physicloy. p. 57" 583. 



