262 THE POPULAR SCIENCE MONTHLY 



feature characteristic of only the more specialized parts of the vertebrate 

 central organs and entirely absent from the invertebrate, but whether 

 this difference really exists or not must remain for future investigation. 



Although it can not be said at present that a synaptic nervous system 

 is the peculiar possession of the vertebrates, there are two important 

 features in which the central organs of these animals differ from those 

 of the invertebrates. In the first place, the central organs of verte- 

 brates exhibit a large prepondernace of long neurones over short ones, 

 and in the second place, they show an enormous increase in the number 

 of association neurones. In an earthworm there are only three long 

 neurones and the rest are short ones; in a crab the long and short 

 neurones are perhaps about equally abundant; but in a vertebrate the 

 long neurones certainly far outnumber the short ones. In any trans- 

 verse section of the spinal cord of one of the higher animals almost 

 all of the white substance in view excepting a thin layer surrounding 

 the ventral horn is made up of systems of long neurones. In this 

 respect the condition in the vertebrates seems to be almost the reverse 

 of that in worms and in consequence transection of their central nervous 

 organs results in profound and extensive degeneration such as is never 

 met with in animals like worms. For this reason the central nervous 

 system of the vertebrate, though giving much evidence of segmentation 

 in its early stages pi growth, is finally a physiological unit such as is 

 realized in no other group of animals, a condition well evidenced by 

 the fact that some of its most recent phylogenetic acquisitions, like the 

 pyramidal tracts of the mammals, may consist of neurones that reach 

 almost from one end of the system to the other. 



The second feature that distinguishes the central nervous organs of 

 vertebrates from those of invertebrates is the enormous development of 

 association neurones. These neurones are present in worms, are nu- 

 merous in arthropods, but are overwhelmingly abundant in vertebrates. 

 Of the white substance seen in the transverse section of the spinal cord 

 almost all except the dorsal columns represent association neurones. 

 Judged from this standpoint there are certainly many more association 

 neurones in the cord than all other kinds taken together. But the asso- 

 ciation neurones are not only the most numerous in the vertebrates; 

 they also constitute the basis of the most significant evolution. The 

 central nervous organs that show the most conspicuous progressive 

 changes in the vertebrates are the cerebellum and the cerebrum, par- 

 ticularly their cortical portions, and when it is remembered that few or 

 no primary sensory or motor neurones contribute to these two organs, 

 but that they are made up of association neurones almost exclusively, it 

 will be seen how enormously important these neurones become. The 

 association neurones in the vertebrates are not only the organs of intri- 

 cate nervous exchange, but in the region of the cerebral cortex they 



