166 GENERAL ZOOLOGY 



fundamental properties. Since Protozoa respond to the same 

 stimuli that activate the nervous system of Metazoa, it is clear 

 that the action of the nervous system as a receptor-effector and 

 coordinating mechanism involves nothing new in principle. 

 Nerve cells may be regarded as cells that have become especially 

 sensitive to stimulation of various sorts and that have also 

 developed the capacity to conduct or transmit nervous impulses 

 at a very rapid rate. A stimulus may be defined as any dis- 

 turbing influence. A nerve impulse results from action of the 

 stimulus. The impulse travels along the nerves by conduc- 

 tion, which is essentially a series or chain of chemical reactions 

 running through the nerve, accompanied by the production of 

 extremely small amounts of heat and carbon dioxide. 



Neuron. — A neuron is a nerve cell. It is the anatomical and 

 to a certain extent the functional unit of the nervous system. 

 Neurons vary considerably in both size and shape in different 

 animals and in different parts of the same animal, but in all 

 cases the neuron consists of at least two regions: (1) a nucleated 

 cell body, and (2) processes extending from the cell body. In 

 higher forms the processes can usually be classified both on a 

 structural and on a functional basis into two groups: (1) dendrons 

 or dendrites, consisting of one or more processes, often branched, 

 that convey nervous impulses toward the cell body; and (2) a 

 single axon, that transmits impulses away from the cell body. 

 Since the neuron as a whole under normal conditions carries 

 impulses in one direction only, it is said to be polarized. Den- 

 drons may be lacking in some neurons, and in others a single 

 dendron may be present and so similar to the axon in form and 

 structure as to be almost indistinguishable anatomically. The 

 drawing of the neuron from the human brain, shown in Fig. 109, 

 illustrates a highly complicated type. The cell body is the pear- 

 shaped region in the center of the figure. , The nucleus cannot be 

 seen because the preparation only shows the entire nerve cell in 

 silhouette. A number of dendrons with collateral branches are 

 attached to the cell body, above and at the sides. The single 

 axon leaves the cell body below and then divides into collateral 

 branches, the main axis of the axon extending downward to join 

 a bundle of nerve fibers. The term "nerve" refers to a bundle 

 of axons or dendrons, or both, bound together by a connective 

 tissue sheath. Axons or dendrons are also known as nerve 



