384 HITCHCOCK'S ANATOMY 



metallic brilliancy, with different shade of color, known as 

 the Tapetum. In horses and cows it has a variegated luster 

 of green and blue, w T hile in the tiger, cat, and whale it is of 

 a silvery brightness. The shape of the pupil in the wolf 

 and dog, which need vision in all directions, is circular ; while 

 the fox and cat, which need a vertical range more than any 

 other, have a perpendicular slit only. In cattle and the more 

 timid grazing animals which obtain their food from the ground, 

 and need to look behind with ease when pursued, the pupil is 

 a horizontal ellipse. Nearly all mammals except man and 

 the apes, have a third lid, which is a transparent membrane 

 that is frequently slid over the ball by a peculiar muscular 

 apparatus. This is called the " haw" or Nictitating Mem- 

 brane, and, especially in birds, serves the purpose of remov- 

 ing impurities from the eye, and also of spreading the secre- 

 tions over it, in order to keep it moist and transparent. In 

 addition to the lachrymal gland, in all animals which have 

 the third lid there is another gland called the Harderian Gland, 

 which prepares another secretion similar to and for nearly the 

 same purpose as the lachrymal. 



7C6. Eyes of Birds — Pccten Marsupium — All birds, 

 without exception, are provided with perfect and well devel- 

 oped eyes. They are always large, and largest in birds of 

 prey. They are but slightly movable themselves, but this 

 want of motion is compensated by the great mobility of the 

 head. The sclerotic coat is strengthened in front by a series 

 of bony plates, fourteen or fifteen in number, interposed be- 

 tween its fibrous layers, a great use of which is to give at- 

 tachment to the special muscular apparatus for adapting the 

 eye to see objects at different distances. The anterior chamber 

 is proportionally larger than in any other animals, so that the 

 iris is far back from the cornea. The iris has different shades 

 of color, commonly yellow or brownish. The yellow color of 



705. What is the tapetum ? What is its color? What is the shape of the pupil in the 

 wolf, clog, and in cattle ? What is the Nictitating membrane? Of what service is the 

 " haw ?'' For what purpose is the Harderian gland ? 706. What is said of the perfection 

 of eyes in birds? How is the sclerotic coat strengthened in them ? 



