THE NERVOUS SYSTEM 



395 



backward ; while invertebrate vision is directly forward. 

 In vertebrates only, the optic nerves cross each other 

 (decussate) in passing from the brain to the eyes ; so that 

 the right side of the brain, e.g., receives the impressions 

 of objects on the left side of the body. 151 



Generally, the eyes of vertebrates are on opposite 

 sides of the head ; but in the flatfishes both are on the 

 same side. Usually, both eyes see the same object at 

 once; but in most fishes the eyes are set so far back, 

 the fields of vision are distinct. The cornea may be 

 flat, and the lens globular, as in fishes ; or the cornea 

 very convex, and the lens flattened, as in owls. Purely 

 aquatic animals have neither eyelids nor tears, but nearly 

 all others (especially birds) have three lids. 152 The pupil 

 is usually round; but it may be rhomb-shaped, as in 

 frogs ; vertically oval, as in crocodiles and cats ; or 

 transversely oval, as in geese, doves, horses, and rumi- 

 nants. Many quadrupeds, as the cat, have a membrane 

 (tapetum) lining the bottom of the eyeball, with a brilliant 

 metallic luster, usually green or pearly ; it is this which 

 makes the eyes of such animals luminous in the dark. 



/ 



2. Instinct and Intelligence 



The simplest form of nervous excitement is mere 

 sensation. Above this we have sensation awakening 

 consciousness, out of which come those voluntary activi- 

 ties grouped together under the name of Instinct ; and, 

 finally, Intelligence. 



The lowest forms of life are completely mechanical, 

 for their movements seem to be due solely to their 

 organization. They are automatons, or creatures of 

 necessity. In the higher animals certain actions are 

 automatic, as breathing, the beating of the heart, the 

 contractions of the iris, and all the first movements of 



