450 TEXT-BOOK OF EMBRYOLOGY. 



(Fig. 398), and is evidently capable of complicated combinations which may 

 be further increased in complexity by the intercalation in the arc of other 

 intermediate neurones. Finally, in the central nervous system certain struc- 

 tures consisting of intermediate neurones are developed which represent the 

 mechanisms for certain coordinations of the highest order. Such are the 

 higher coordinating centers (suprasegmental structures of Adolf Meyer) . 



As a result of the preceding, it follows that in seeking the explanation for 

 various nervous structures there must always be kept in mind, first, their correla- 

 tion with peripheral structures and, second, the degree of development of the 

 central coordinating mechanism represented by the intermediate or central 

 neurones. The most important features common to the nervous systems of 

 all Vertebrates owe their uniformity either to a corresponding uniformity in 

 the peripheral receptors and effectors, or to a uniformity in the coordinations of 

 the stimuli received and given out by the central nervous system. Variations 

 in structure are due to variations of either the peripheral or central factor above 

 mentioned. In the lower Vertebrates the former factor plays a relatively more 

 important part than in the higher Vertebrates, the central apparatus being 

 simpler; while in the development of the higher vertebrate nervous systems the 

 dominating factor is the increasing complexity of the central mechanism. The 

 superiority of the nervous system of man does not consist, in the main, of supe- 

 riority in sense organs or motor apparatus, but in the enormous development of 

 the intermediate neurone system. 



GENERAL PLAN OF THE VERTEBRATE NERVOUS SYSTEM. 



The Vertebrate is an elongated bilaterally symmetrical animal progressing 

 in a definite direction, primitively perhaps by alternating lateral contractions 

 performed by a segmented lateral musculature. Associated with these char- 

 acteristics are the bilateral character of the nervous system and its transverse 

 segmentation, shown by its series of nerves, a pair to each muscle segment. 

 The definite direction of progression involves a differentiation of the forward 

 extremity of the animal, such as the location there of the mouth and respiratory 

 apparatus and the development there of specialized sense organs, the nose, eye, 

 ear, lateral line organs, and taste buds, which increase the range of stimuli 

 received by the animal and thereby render possible a greater range of responsive 

 activities in obtaining food and in reproduction. As a natural outgrowth 

 of these specializations, the highest development of the central coordinating 

 mechanism also takes place at the forward end or head. This concentration 

 and development of various mechanisms in the anterior end is usually termed 

 cephalization, and is a tendency exhibited also by various groups of Inverte- 

 brates in which the same general conditions are present. 



The typical vertebrate nervous system, then, consists of a bilateral central 



