CHAPTER II 



THE NERVOUS FUNCTIONS 



THE body is composed of organs and tissues, the organs being 

 parts with particular functions to perform and the tissues being 

 the cellular fabric of which the organs are composed. The 

 tissues (which must be studied microscopically) are classified, 

 sometimes in accordance with the general functions which they 

 serve, such as the nervous and muscular tissues, and sometimes 

 with reference to the forms and arrangements of their compo- 

 nent cells. An illustration of the latter method of treatment is 

 furnished by the epithelial tissues, which are thin sheets of 

 cells, sometimes arranged in one layer (simple epithelia), some- 

 times in several layers (stratified epithelia). Epithelial tissues 

 may perform the most diverse functions. 



All living substance (protoplasm) possesses in some measure 

 the distinctive nervous functions of sensitivity and conductivity, 

 that is, it responds in a characteristic fashion to certain exter- 

 nal forces (stimuli), and when thus stimulated at one point the 

 movement or other response may be effected by some remote 

 part. This last feature implies that some form of energy is 

 conducted from the site of the stimulus to the part moved. 

 Ordinary protoplasm also possesses the power of correlation, 

 that is, of combining a number of individual reactions to stimu- 

 lation in diverse special adjustments. 



The one-celled animals and all plants lack the nervous sys- 

 tem entirely ; nevertheless they are able to make highly complex 

 adjustments. The leaves, roots, and stems of the higher plants 

 have individual functions which are, however, bound together 

 or integrated into a very perfect unity. In animals, as con- 

 trasted with plants, we see a further differentiation of parts of 

 the body for special functions, and at the same time a more per- 

 fect correlation of part with part and integration of the whole 

 for rapid and diversified reactions of the entire body. The 



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