36 INTRODUCTION TO NEUROLOGY 



innate structure, or there may be some small measure of individual modifi- 

 ability, but when the modifiability comes to be the dominant characteristic, 

 so that the result of the stimulus cannot be readily predicted with mechan- 

 ical precision, the process may be called association. The intelligent types 

 of reaction and all higher rational processes belong here, and the cerebral 

 cortex is the chief apparatus employed. 



The boundaries between the three types of centers just distinguished 

 are not always sharply drawn, especially in their simpler forms, though in 

 general they are easily distinguished. The mechanisms of coordination 

 are neurologically simpler than those of correlation and association, and in 

 general they are developed in the more ventral parts of the brain and 

 spinal cord, that is, below the limiting sulcus of the embryonic brain (p. 120). 

 The correlation and association centers are developed in the more dorsal 

 parts of the brain and cord, and the greater part of the thalamus and cere- 

 bral hemispheres is composed of tissues of this type. Nevertheless, the dis- 

 tinctions here drawn are fundamentally physiological rather than anatom- 

 ical, and coordination centers may be developed in the dorsal parts of the 

 brain, as in the case of the cerebellum and probably also the corpus striatum 

 of mammals (though not the striatum of lower vertebrates). 



Summary. The functions which characterize the nervous 

 system have been derived from those of ordinary protoplasm 

 by further development of three of the fundamental protoplas- 

 mic properties viz., sensitivity, conductivity,, and correlation. 

 The most primitive form of nervous system known is diffuse and 

 local in its action, but in all the more highly developed forms the 

 chief nervous organs tend to be centralized for ease of general 

 correlation and control. Most of the types of nervous systems 

 found in the animal kingdom are represented in two distinct and 

 divergent lines of evolution, one adapted especially well for the 

 reflex and instinctive mode of life and found in the worms, in- 

 sects, and their allies, and the other found in the vertebrates and 

 culminating in the human brain with its remarkable capacity 

 for individually acquired and conscious functions. 



LITERATURE 



BARKER, L. F. 1901. The Nervous System and Its Constituent Neu- 

 rones, New York. 



CHILD, C. M. 1911. The Regulatory Processes in Organisms, Journal of 

 Morphology, vol. xxii, pp. 171-222. 



EDINGER, L. 1908. Th^ Relations of Comparative Anatomy to Compar- 

 ative Psychology, Jour. Comp. Neur., vol. xyiii, pp. 437-457. 



HERRICK, C. JUDSON. 1910. The Evolution of Intelligence and Its 

 Organs, Science, N. S., vol. xxxi, pp. 7-18. 



. 1910. The Relations of the Central and Peripheral Nervous Systems 

 in Phylogeny, Anat. Record, vol. iv, pp. 59-69. 



