194 The Vertebrate Organ Systems 



The nervous system is aided in its integrative activities by the 

 work of the endocrine system. The function of this latter system will 

 be considered in a later chapter. 



THE ANATOMY OF THE NERVOUS SYSTEM 



Functionally the nervous system is a unified whole ; for the sake 

 of discussion, however, it is often arbitrarily divided into three struc- 

 tural parts. These are the central nervous system consisting of the 

 brain and spinal cord, the peripheral nervous system formed of the 

 cranial and spinal nerves, and finally the autonomic nervous system 

 consisting of the sympathetic and parasympathetic functional divisions. 

 Both structurally and functionally these divisions overlap. 



The Structural Unit of the Nervous System. — The unit of struc- 

 ture of the nervous system is the nerve cell or neuron (Fig. 57), a more 

 or less stellate-shaped cell. The large central mass of this cell is known 

 as the cell body. Within the center of the cell body is the nucleus which 

 is surrounded by the dark-staining Nissl's granules. The functioning 

 of these granules is somewhat obscure, but possibly they serve as re- 

 serve food particles. Extending from the main cell body are projec- 

 tions : the dendrites and the axons. Structurally these appear nearly 



Fig. 57. — The nerve cell or neuron. 



identical ; functionally, however, they differ. They are able to carry 

 nerve impulses in but one direction. The dendrites carry impulses 

 into the cell body and are known as afferent processes. The axons 

 which carry impulses away are known as efferent processes. A single 

 neuron may have several dendrites, but there is only a single axon. 

 Both processes are able to make connections with other nerve processes, 

 and in some cases the ends of dendrites may function in sense organs 

 as receptors. While some of these processes are very long, others are 

 very short. A long one, for example, may extend from the spinal cord 

 to the muscles of the foot ; a short one may simply connect with the 

 processes of a neighboring cell. Contacts with adjoining nerve cells 



