430 MAN BECOMES THE CONQUEROR OF THE WORLD 



Compare, part by part, the brain of the frog with a model of the brain 

 of man. In what respect is a frog's nervous system like that of man? 

 How does it differ? Write a description, comparing the nervous sys- 

 tem of the frog with your own, using charts and models as a guide. 



The sense organs of man. We have seen that simpler forms 

 of life perform certain acts because outside forces acting upon 

 them cause them to react. All many-celled animals, including 

 man, are put in touch with their surroundings by what we call 



special sense organs. The senses of 

 man, besides those we commonly 

 know as sight, hearing, taste, smell, 

 and touch, are those of temperature, 

 pressure, and pain. It is obvious that 

 such organs, to be of use, must be at 

 the outside of the body. Thus we 

 find eyes and ears in the head, and 

 taste cells in the mouth, cells in the 

 nose for smelling, and others in the 

 skin which are sensitive to heat or 

 cold, pressure or pain. 



The nervous system. In the ver- 

 tebrate animals, including man, the 

 nervous system consists of two divisions. One, including the 

 brain, spinal cord, and nerves, makes up the central nervous 

 system. The other division, called the autonomic nervous system, 

 consists of small collections of nerve cells called ganglia. These 

 ganglia are mostly included in two chains parallel to the spinal 

 cord. This system transmits stimuli from the central nervous 

 system to the heart, glands, and muscles of the internal organs. 



Strangely enough, we do not see with our eyes or taste with 

 our taste cells. These organs receive the stimulations which are 

 sent inward by means of a complicated system of greatly elongated 

 cell structures, until the sensory message reaches an inner station, 

 in the central nervous system. We see and hear and smell in our 

 brains. 



Neurons. The unit of structure of the nervous tissue is a cell, 

 called a neuron. It is a mass of protoplasm containing a nucleus. 



Some parts of the body are more 

 sensitive to certain stimuli than are 

 others. The diagram on the left 

 shows an organ that is concerned in 

 the sensation of touch; the one on 

 the right, concerned in the sensation 

 of pressure. 



