KAT, A KKPRESENTATIVE MAMMAL 133 



protoplasm. As nervous tissue develops, these properties are spe- 

 cialized, and the efficiency of the organism increases as the awareness 

 of the environment increases. The development of sense organs in- 

 creases and enhances this awareness. Although animals without nerv- 

 ous systems exhibit excitation, conduction, and to some extent cor- 

 relation, these functions expand in range and power when the sense 

 organs and nerve pathways become available. As complexity and 

 range in the mechanism of the nervous system increase, centers of 

 correlation arise for a more precise analysis of the excitation in order 

 that an appropriate reaction will result. Thus, means is provided 

 for the organism to relate itself better to the environment. 



The fundamental units of the nervous system consist of a sensory 

 neuron and a motor neuron. The neuron (nerve cell with all of its 

 processes) is the unit of structure in nervous systems, from the sim- 

 plest to the most complex, even to include those of rats and men. Two 

 or more neurons may be associated to function as a reflex arc, and this 

 arc rather than the single neuron becomes the functional unit. In 

 higher forms of vertebrate animals there is more and more organiza- 

 tion and concentration of neurons which brings about increasing 

 prominence of the brain portion of the system. In man, cerebral ac- 

 tivity takes a dominant part in the functions of the entire system; 

 however, there has been no loss of significance or extent of develop- 

 ment and activities of the other parts. The entire system controls 

 and coordinates the activities of the other parts of the body. 



The entire system of the rat and other higher vertebrates is usually 

 divided into four general divisions: (1) central, including the brain 

 and spinal cord; (2) peripheral nerves, including cranial and spinal 

 nerves; (3) sense organs, as eyes, ears, tactile corpuscles, taste buds, 

 and olfactory structures; and (4) autonomic (sympathetic), includ- 

 ing the two trunks of ganglia. 



Central Nervous System. — The central portion of the system is 

 tubular, having developed as a simple neural tube. This portion of 

 the system is covered by three membranes called meninges. The 

 outer tough membrane is the dura mater; beneath this is the delicate 

 arachnoid layer ; and adhering to the surface of the nervous tissue is 

 the vascular pia mater. The anterior region of this central portion 

 is the brain. It is considerably broadened and thickened, presenting 

 five divisions. From anterior to posterior these divisions are: tel- 

 encephalon, diencephalon, mesencephalon, metencephalon and mye- 

 lencephalon. 



