Johnston, MorpJiology of the Head. 183 



structures. The coordination of action between systems of 

 organs having different functions is effected through central 

 connections between these functional divisions. To what ex- 

 tent direct, simple or short connections between the centers of 

 different divisions serve this purpose is not yet fully understood. 

 But at least in some cases it appears that the divisions remain 

 separate as far as their tertiary tracts and coordination is 

 effected through special structures. It follows that these longi- 

 tudinal divisions of the nervous system are fundamental in 

 character and are much more significant than the transverse 

 divisions (fore-brain, hind-brain). Each functional division is 

 represented in successive segments by serially homologous or- 

 gans, while the transverse divisions of the brain are for the 

 most part merely mass portions which have little segmental 

 value. It remains to show the bearing of this view of the 

 nervous system upon the problems of head morphology. 



A serious attempt can no longer be made to determine the 

 segments of the vertebrate head by a study of the structure 

 and development of one system of organs alone or chiefly. 

 The method is limited because the set of organs selected for 

 study is not coextensive with the whole organism. One quasi 

 exception to this exists. Owing to the necessities of the case 

 the nervous system is represented in every segment of the or- 

 ganism. Each system of organs has its representation in the 

 nervous system ; if we can read aright, in the segmentation of 

 the nervous system we read the segmentation of the organism. 



Reasoning in some such way as this, some authors have 

 attempted to determine the segmentation of the head by means 

 of the central nervous system alone. While the central nerv- 

 ous system at an early stage of development may show the 

 number of segments of the whole head, it can show little as to 

 the relations and arrangement of other organs, what organs are 

 present in given segments, etc. Further, any study of seg- 

 mentation from the nervous system alone is met by the diffi- 

 culty that, although some part of the nervous system must be 

 present in each segment, one or more of its main divisions may 

 be absent. What part of the nervous system is represented in 



