AND PHYSIOLOGY. 



385 



this part in the owl is owing to the presence of cells contain- 

 ing fat. A curious structure is found in the eye of birds that 

 is found in no other animal. It is called the " Pecten Mar- 

 supium," and consists of a layer of blood-vessels arranged in 

 several plications varying greatly in size. It extends some- 

 times as far forwards as the lens, and is covered by black pig- 

 ment. Its probable use is that of absorbing rays that enter 

 the eye obliquely, and thus rendering sight from a forward 

 direction more definite and sharp. The membrana nictitans 

 and the Harderian glands are present in these animals in their 

 fullest perfection. 



707. Eyes of Reptiles— All reptiles have two eyes. In 

 most, especially in serpents, they are small in comparison 

 with the size of the body ; in the frog, however, in the gechos 

 and chameleons they are large. In some they are covered by 

 the skin. In the serpents there are no eyelids, but the skin 

 which covers them is kept moist by a lachrymal gland, and 

 thus performs the service of a lid. In other reptiles there 

 are three lids, the middle one of which moves at right angles 

 to the other two. The lachrymal gland often is very large. 

 In some turtles and lizards there is a ring of bony plates 

 which supports the eyeball. The lens has different degrees 

 of sphericity in different animals. 



708. Eye of Fishes — Pigmentary Spots— The eyes of 

 fishes are remarkable for the great size and spherical form of 

 the lens, w r hich is necessary in order to give sufficient refrac- 

 tion to the rays of light that come from the dense medium in 

 which they live. The size of the lens varies greatly, being 

 largest in the bony fishes and smallest in those which lie 

 buried in the mud like the eel. The lowest stage, however, 

 is seen in the amphioxus, a condition which greatly resembles 



What makes the yellow color of the iris in birds? What is the pecten marsupium? 

 What is its use ? 707. What is said of the eyes of reptiles? Wliat of the bony platea 

 in the eyeball? 708. What characterizes the eyes of fishes? What ones have the 

 largest? 



