288 STUDIES IN PHYSIOLOGY 



come out of their burrows except at night, hence, although 

 they have no eyes, they can distinguish light from dark- 

 ness. These animals take in certain substances for food 

 and refuse to take others, which indicates that they can 

 taste or smell, or both. The contraction of the various 

 muscles in the body wall, the movement of the bristles, the 

 rhythmic pulsations of the blood vessels, and the action of 

 the various glands, are all controlled by the nervous system. 

 We know, too, that when an earthworm is cut in two, both 

 pieces can move about for a time. Hence the ganglia in dif- 

 ferent parts of the body can act more or less independently. 

 The posterior region, however, soon dies. But the anterior 

 part, which has the brain, can often develop new segments 

 and in time become a complete worm. 



The Nervous System of Invertebrates and of Vertebrates. 

 In the earthworm, as we have just seen, the central nervous 

 system is situated in the ventral region of the body. The 

 same is true in starfishes, clams, lobsters, and insects. In- 

 deed, in all invertebrates we find most of the ganglia (when 

 they are present at all) to be ventral. Vertebrates, on the 

 other hand, have a dorsal brain and spinal cord wholly or 

 largely inclosed within a skull and spinal column, and most 

 vertebrates, too, have two chains of sympathetic ganglia 

 which extend along the dorsal region of the body cavity on 

 each side of the spinal column, and which largely control 

 the organs of digestion, circulation, and excretion. But while 

 the general plan of the nervous system in all vertebrates 

 is the same, striking differences occur in detail. This will 

 be clearly shown by 



A Comparison of Vertebrate Brains. On p. 289 are repre- 

 sented the brains of the salmon (fish), the frog (amphibian), 

 the alligator (reptile), the pigeon (bird), and the dog and 

 man (mammals). Thus we have a representative form of 

 each of the five groups of vertebrates, and for convenience 

 in comparison the brains, in spite of their great differences 

 in actual volume, are represented as though they were of the 



