CHAPTER XXVIII 



MAN (Continued) 

 THE MIND OF MAN 



Man excels other animals in his combination of erect 

 attitude, opposable thumb, unusual brain development, 

 and powers of speech. The chief characteristic which has 

 enabled him to outstrip all other competitors in his reason- 

 ing power. This of course has its seat in the nervous sys- 

 tem and is dependent upon growth and cell-division for 

 its full development. The nervous system of man, like 

 that of all vertebrates, is formed by the turning in of a 

 groove which is finally pinched off to form a tube along the 

 dorsal side of the body (Fig. 91). As development pro- 

 ceeds (Fig. 104) tHe wall of the tube thickens and becomes 

 folded in places; long fibers grow out from its nerve cells to 

 all parts of the body and collect in bundles to form nerves. 

 Sense organs develop, usually on or near the outside of the 

 body, and become connected by nerve-cell fibers with the 

 central tube. All the great nervous structure of an adult 

 man has arisen by growth and cell-division from the zygote, 

 formed when an egg cell and spermatozoon fused. 



In its fully formed condition the nervous system consists 

 of three classes of organs: (1) receptors, (2) effectors, and 

 (3) adjusters. According to Herrick there are, instead of 

 "five senses, " about twenty-three kinds of receptor organs* 

 in the human body which receive stimuli from outside or 

 notify the central nervous system of conditions within. 



* Separate receptors are stimulated by: touch and pressure, cold, heat, 

 pain, chemicals, sounds, light, odors, muscle "tonus," tendon "tonus," 

 hunger, thirst, nausea, suffocation, disturbances in circulation, sexual 

 stimuli, distention of cavities (stomach, bladder, etc.), obscure abdominal 

 changes, and tastable substances. 



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