NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 5 



if it is to be aware of what concerns its welfare in the outside world ; 

 and it must also have paths of communication from the nerve centers 

 to its muscles and other organs, if it is to control its activities in 

 accord with the information it receives from without. These bonds 

 extending between the nerve centers and the rest of the organism 

 are the nerve trunks, which, all together, constitute the peripheral 

 nervous system. 



THE STRUCTURE OF NERVOUS TISSUE 



All nerve fibers are connected with nerve cells ; according to the 

 modern conception of the morphology of nervous tissue, nerve fibers 

 are in all cases prolongations of nerve cells. A nerve cell, in the 

 broadest sense, then, consists of the cell body, or cyton (fig. 2, Cy), 

 together with all of its nerve processes. The entire system is called 



Fig. 2. — Diagram of a nerve cell, or neuron. 



a, dendrons; Axn, axon; b, c, terminal arborization; Col, collateral; 

 Cy, nerve cell body, or cyton. 



a neuron. A nerve cyton is said to be unipolar, bipolar, or multipolar, 

 according to the number of processes that arise from it. Of the cell 

 processes, several may be short and branching, but one is usually 

 a long, smooth, continuous axis with few branches. The first are 

 known as dendrons, or dendrites (a), the second is the axon, or 

 neurite (Axn), and constitutes the true nerve fiber from the cell. 

 Branches given off from an axon are distinguished as collaterals 

 (Col). The axons, collaterals, and dendrons terminate in fine branches 

 (b), often spoken of as arborisations. 



The principal function of nervous tissue is the transmission of 

 stimuli. Stimuli originate either from influences outside the body 

 (sensory stimuli) or from influences within the body. According 

 as the stimuli travel along the nerve fibers toward or away from 

 the nervous centers, the fibers are said to be afferent fibers or 

 efferent fibers. The fibers which receive stimuli directly, either from 

 without or from within, and transmit them to the nerve centers, 

 constitute the sensory nervous system. The efferent nerves and 



