222 NERVOUS SYSTEM OF VERTEBRATES. 



of time involving adjustment and readjustment of muscles with 

 reference to some whole act destined to reach a given end, the 

 better organized brain centers are necessary. It is the function of 

 relating several simple actions with reference to some common 

 end, the co-relation of activities, which these brain centers serve. 



The degree of complexity of the activities controlled by the 

 centers of correlation in various animals is directly paralleled by 

 the complexity of the brain itself. The efforts at escape made by a 

 normal frog when seized are much more complex and long con- 

 tinued than those of a frog whose brain has been destroyed. To 

 the human observer, however, the efforts of the normal frog are 

 very simple. The frog has no cerebral hemispheres related to 

 somatic sensation and somatic movement. The efforts of a 

 mammal with its^cerebral cortex are enormously more complex 

 and may involve keen observation, connected effort through 

 relatively long periods, employment of indirect means, etc. 



In the lowest vertebrates, cyclostomes, a large part of the sub- 

 stantia reticularis of the brain remains in its primitive indifferent 

 condition; few special nuclei are developed and the activities of 

 the animals are correspondingly simple. To any stimulus that 

 may come the animal can respond only in a very limited number 

 of ways. To two similar stimuli little or no difference in response 

 is to be expected. In the whole hindbrain region no special nuclei 

 in the substantia reticularis have been found. Even the secondary 

 gustatory nuclei are so little developed that they have not yet been 

 seen. The cerebellum is wholly unspecialized. The tectum 

 opticum and the nucleus of the tractus habenulo-peduncularis 

 in the mesencephalon, the nucleus habenulae, the nucleus of the 

 posterior commissure and the inferior lobes in the diencephalon, 

 and the striatum and epistriatum in the telencephalon probably 

 comprise all the special nuclei at present known in the cyclostome 

 brain. These will be considered in connection with the brain 

 of true fishes below. 



In the different classes of vertebrates the process of differ- 

 entiation of many of the special centers which are well marked 

 in the mammals is seen in various stages of completeness. In 

 some selachians and ganoids parts of the brain are little more 



