Johnston, Functional Divisions of Nervons System. 99 



and an attempt to compare the chief and most fundamental 

 divisions of the mid brain with those of the cord must make 

 use, if possible, of the oldest part of the mid brain. A com- 

 parison of the tectum of Petromyzon with that of higher fishes 

 and amphibia shows that the secondary center for the somatic 

 sensory column is nearly as well developed in Petromyzon as in 

 the higher forms, while the elements belonging to the optic ap- 

 paratus are few and poorly differentiated, corresponding to the 

 slight development of the eyes. It therefore seems to be a 

 warranted assumption that the nucleus of the internal arcuate 

 fibers in the tectum represents the most fundamental structure 

 in the dorsal part of the mid brain. The nervous elements of 

 this nucleus more nearly resemble the elements of the somatic 

 sensory centers in their size, form, stage of development, and 

 their arrangement, than those of either of the other columns. 

 The suggestion may be ventured that the secondary center in 

 the tectum was originally a more rostral portion of the somatic 

 sensory column, that the internal arcuate fibers are to be inter- 

 preted as crossed connectives between distant segments of the 

 somatic sensory column, which are probably represented 

 at other levels by the external arcuate fibers. This im- 

 plies the following conception of the arrangement of the 

 somatic centers in primitive vertebrates : certain fibers served 

 as crossed connectives from lower to higher segments of 

 the somatic sensory column, while others probably made 

 indirect connections with the motor centers by way of the 

 commissural and tract cells. There has been a gradual tendency 

 for the connectives to pass forward to the most rostral part of 

 the somatic sensory column and as a result this region has be- 

 come differentiated as a special secondary nucleus, which receives 

 no sensory root fibers but only these connnectives. At the same 

 time it has come about that the fibers which make conections be- 

 tween the sensory and motor centers arise only from this special 

 region. The result is to give a more complex and efficient dis- 

 tributing and coordinating mechanism ; a more highly organ- 

 ized apparatus for somatic reflexes, which is more capable of 

 producing definite movements adapted to specific ends. This 



