SYSTEMATIC ANALYSIS OF TISSUES AND FLUIDS. 145 



tion ; but the general plan and idea have been correctly 

 exhibited. 



578. THE EVOLUTION OF NERVOUS SUBSTANCE, es- 

 pecially the gray or active part, is the most time-taking, 

 elaborate, and expensive of all the tissues. 



579. PLANTS PRODUCE THE COMPONENTS OF NER- 

 VOUS TISSUE in comparatively very small quantities, and 

 if the nervous tissue is very active they must be selected 

 with care. 



580. THE COMPONENTS OF NERVOUS TISSUE abound 

 more in certain kinds of meat-diet than they do in plants, 

 as would naturally be the case. 



581. THE ELABORATE PROCESS OF PERFECTING THE 

 NERVOUS TISSUE, combined with the necessity for its 

 constant operation during waking hours, makes it neces- 

 sary that it should have periods of complete repose, as in 

 sleep, during which no process requiring its action should 

 take place. 



582. SINCE THE PROCESS OF SECRETION is VEGETA- 

 TIVE, it can go on during sleep, at a moderate degree, 

 without the influence of the nervous tissue ; and as the 

 Heart and the organs concerned in breathing rest three 

 fourths of the time, the process of producing heat and 

 circulating Blood may take place without the necessity 

 of long periods of complete inactivity in their ganglia. 



583. PROXIMATE OR ORGANIC PRINCIPLES, OR OR- 

 GANIC ELEMENTS, are the names given to all the com- 

 ponents produced in plants or in the Body adapted to 

 its nutrition. 



584. WHEN THE ORGANIC ELEMENTS ARE ANALYZED 

 by the chemist, they are found to be composed of at least 

 thirteen, usually sixteen, and sometimes, but probably 

 accidentally, nineteen simple or chemical Elements. 



585. SIMPLE OR CHEMICAL ELEMENTS are so called 

 because they are the simplest substances known, having 



578. What said ? 579. What do ? 580. What said ? 581. What said ? 

 582. What inference ? 5S3. What are ? 584. What shown ? 585. What are ? 



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