142 



PRINCIPLES OF ANIMAL BIOLOGY 



and innervate the organs of the head and neck and many organs of the 

 trunk. The distribution of these nerves is practically the same in all the 

 craniate vertebrates (those possessing crania or skulls). From the spinal 

 cord paired nerves originate, corresponding to the somites of the body 

 as indicated by the vertebrae. Each spinal nerve originates by two 

 roots, dorsal and ventral (Fig. 112). The dorsal root just median to its 

 union with the ventral root expands into a ganglion which is composed 

 of nerve cells whose processes make up much of the nerve. Originating 

 in the brain (Fig. Ill) and extending down on either side of the spinal 

 cord are two strands of nervous tissue which are connected by nerves 

 with the dorsal root ganglia. These strands with their branches and 

 ganglia, which may be widely separated from the strands, comprise the 

 sympathetic nervous system. * 



cfrgf 



Fro. 112. — Diagram showing origin of spinal nerve by two roots, off, afferent fibers; 

 dh, dorsal horn of the gray matter of the spinal cord; drg, dorsal root ganglion; eff, efferent 

 fibers; vh, ventral horn of the gray matter of the cord. 



Neurons. — The unit of structure of the nervous system is the neuron 

 or nerve cell. These cells are of various shapes and sizes, a few of which 

 are illustrated in Fig. 78. The neuron consists of a cell body from which 

 commonly arise short branching processes, the dendrites, and a single 

 unbranched process, the axon. The latter is often very long and may be 

 surrounded by a sheath (Fig. 119). Neurons are brought into relation 

 with other neurons by means of their dendrites and axons, which may 

 come directly in contact with the bodies of other neurons or indirectly 

 through their dendrites. The ends of the processes may be in contact 

 with tissues. 



References 



Dahlgren, U., and W. A. Kepner. A Textbook of Principles of Animal Histology. 



KiNGSLEY, J. S. Textbook of Vertebrate Zoloogy, Part I. 



Parker, T. J., and W. A. Haswell. Textbook of Zoology, Vols. I and II. 



