XXVIII. THE NERVOUS SYSTEM AND ORGANS OF SENSE 



Frohlem Z,V. ^ study of the nervous system, reactions to 

 stimuli, and habit formation. (.Laboratory Manual, Prob. LV.) 



Divisions of the Nervous System. — The control of a number of 

 activities for the attainment of a definite end is the function of the 

 nervous system in the lowest as well as the highest of animals. 

 In the vertebrate animals, the nervous system consists of two 

 divisions. One includes the brain, spinal cord, the cranial and 

 spinal nerves, which together make up the cerebrospinal nervous 

 system. The other division is called the sympathetic nervous sys- 

 tem. The activities of the body are controlled from nerve centers 

 by means of fibers which extend to all parts of the body, there 

 ending in the muscles. The brain and spinal cord are examples 

 of such centers, since they are largely made up of nerve cells. 

 Small collections of nerve cells, called ganglia, are found in other 

 parts of the body. These nerve centers are connected, to a greater 

 or less degree, with the surface of the body by the nerves which 

 serve as pathways between the 

 end organs of touch, sight, taste, 

 etc., and the centers in the brain 

 or spinal cord. Thus sensation is 

 obtained. 



Nerve Cells and Fibers. — A nerve 

 cell, like other cells in the body, is a 

 mass of protoplasm containing a nu- 

 cleus. But the body of the nerve cell 

 is usually rather irregular in shape, 

 and distinguished from most other cells 

 by possessing several delicate, branched 

 protoplasmic projections called den- 

 drites. One of these processes, the 

 axis cylinder process, is much longer 

 than the others and ends in a muscle 



399 



A nerve cell from the brain of a 

 monkey, showing a great number 

 of tiny protoplasmic projections 

 or dendrites. 



