FISHES 245 



cord enlarges to form a brain, entirely different in structure from 

 the so-called brains of the lower forms, in that it has developed 

 separate regions for different functions. The fish's brain consists 

 of five principal parts. Beginning at the anterior, come the olfactory 

 lobes from which the nerves of smell extend to the nostrils. Pos- 

 terior to these, and considerably larger, are the two lobes of the 

 cerebrum, which control the voluntary muscles of the animal. 

 The largest parts of the brain are the two optic lobes connected 

 directly with the eyes and concerned, of course, with the sense of 

 sight. Behind them comes the cerebellum, and finally the enlarged 

 end of the spinal cord, the medulla, both of which have to do with 

 regulating muscular action and the work of the internal organs. The 

 medulla is also a region from which branch many important nerves. 



The brain as a whole, compared with other vertebrates, is not 

 highly developed. The cerebrum, the center of voluntary control, 

 is actually smaller than the optic lobes, and the whole brain does 

 not fill the cranium or skull cavity, which is partly occupied by a 

 protective liquid. It is only when compared with the invertebrate 

 forms, that the real advance of the fish brain can be realized. In 

 them there were no special parts for separate uses, no division of 

 labor or specialization, and so a highly developed instinct was the 

 best such a brain could achieve. 



In the vertebrate, the development of specialized parts of the 

 brain, though very primitive at first, paved the way for a cerebrum 

 which would exceed all the other brain regions in bulk, and control, 

 not only voluntary motion, but thought and reason, as well. So 

 when studying the simple brain of the fish, do not forget that it 

 contains the possibilities of great advance, and is to be the line 

 along which the highest vertebrate development will be attained. 



Air Bladder. Another organ, simple in the fish, but which has 

 a great future before it, is the air bladder which is found in most 

 species. This consists of a thin-walled elliptical sac, located in 

 the dorsal part of the visceral cavity and sometimes connected 

 with the throat by a tube. Its function is to assist the fish in 

 maintaining a level in the water; by contraction of its walls the 

 fish can sink, and by expansion, rise without other effort. 



