D. Nervous Tissues (Figs. 16 and 17) 



NERVOUS 



Nervous tissues are composed of nerve cells (neurons), 

 each with a single nucleus. The cells vary greatly in 

 size and shape, as well as in their cytoplasmic processes 

 (axon and dendrite). The axon (axis cylinder) is long 

 and usually unbranched except for occasional side col- 

 laterals. The dendrites (Gr. dendron, tree) are usually 

 much branched, especially near the neuron, although 

 branches appear to be absent in some instances. The 

 dendrite carries impulses toward the neuron, while the 

 axon carries them away. The minute gap between con- 

 secutive neurons regulates the transmission of impulses 

 between them and is known as the synapse (sin' aps) 

 (Gr. synapsis, union). Neurons, depending on the num- 

 ber of cytoplasmic processes, may be classed as unipolar 

 (one), bipolar (two), multipolar (more than two proc- 

 esses). A nerve consists of the processes of nerve cells 

 united into a sort of "cable." A ganglion is an enlarge- 

 ment composed of the nerve cells and serves as a center 

 of nerve influence outside of the central nervous system. 



Nerve fibers (processes of neurons) may be classed as (a) 

 medullated (myelinated) when they are surrounded by 

 a noncellular, fatty medullary sheath (myelin sheath) 

 with constrictions at intervals by the nodes of Ranvier and 

 (b) nonmedullated (unmyelinated) , in which the nerve 

 fiber lacks the medullary sheath. A thin nucleated 

 neurilemma (nu ri -lem' a) (Gr. neuron, nerve; lemma, 

 cover) may cover certain nerve fibers, such as peripheral 

 nerves going to skin, muscles, viscera. A brain (cerebral 

 ganglion) and spinal cord are composed of various kinds 

 of neurons with processes of various types. 



Parts of the Nervous System (Figs. 16 and 17) 



Central nervous 

 system 



Peripheral 

 nervous 

 system 



[Cerebrum 

 Cerebellum 

 Brain -j Midbrain 



Medulla oblongata 

 *■ Pons varolii 

 Spinal cord 

 f Cranial (brain) nerves and their end organs 

 Spinal nerves and their end organs 

 Sympathetic nerv^ous system with its various 

 subdivisions 



l^-Demdnte 



/Jijjl <^rQnu]e 



y '' ^MucYeas 



Axon //eurolemma 



yAxon brush 



/ 



;^''. ■"■"■ ' -" ^^^^^wHwTi*^^^i?*^Mii .j.j . ..: i)i ; 5^g 



Medullary Jheabh /Jode oi Ranvier 



Fig. 16. — Nervous tissue. A nerve cell or neuron is shown much enlarged and 



somewhat diagrammatically. 



