CHAPTER VI. 

 THE NERVE TISSUE. 



All cells are to a certain extent irritable and conductive; that is, 

 they receive stimuli from external sources and transform them into 

 impulses which are conducted to a portion or the whole of the cell 

 to stimulate some reaction- by that portion or by the entire cell. 

 In unicellular animals and the simplest metazoans, no special 

 organization appears to be developed to carry on these fundamental 

 functions of protoplasm. x\mong the metazoans generally, how- 

 ever, an association of special cells has evolved to form the nerve 

 tissue, which functions primarily as a receiver of stimuli and con- 

 ductor of impulses. The vital unit of this tissue is the nerve cell, 

 or neuron, which takes on varied forms but invariably has one or 

 more cytoplasmic processes making contact with closely adjacent 

 or more remote cells or tissues of the body. Typically, each cell 

 has a large nucleus sin-rounded by a cytoplasmic mass from which 

 slender processes grow out for varying distances to form nerve 

 fibers. Each neuron is a separate unit, but the processes of one 

 cell come into contact with others at synaptic junctions, or synapses, 

 so that impulses pass from one nerve cell to another, and by chains 

 of such cells impulses may be conducted over considerable distances 

 to finally effect responses in other cells or tissues. Little is known 

 concerning the exact nature of stimuli, or how they are transformed 

 into impulses, or how the latter are transmitted along nerve cells, 

 but the essential part played by these cells in coordinating the other 

 tissues of the organism has been proven repeatedly by careful 

 experimentation. 



The nerve cells and their processes are organized into organ 

 centers such as the lirain, spinal cord, and ganglia, but the jh'o- 

 cesses alone form the nerves which are organized into an intercon- 

 necting system associating the ^'arious tissues and organs with the 

 nerve center and making possible integrated action. 



HISTOGENESIS OF NERVE TISSUE. 



The foundation of all the nerve tissue appears in the de\'eloping 

 embryo as a thickened region of ectoderm, the neural plate, along 

 the mid-dorsal line. Following rapid and unequal growth of the 



