MAMMALIA— PHYSIOLOGY 



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include the seventh (facial), eighth (auditory), the ninth (glosso- 

 pharyngeal) and the tenth (pneumogastric). 



Origin and Structure of the Neuron. — A neuron consists of a cell 

 body with all of its processes. The entire nervous system is com- 

 posed of cells with one or more processes. These cells develop early 

 in embryonic life from certain ectodermal cells (neuroblasts) of the 

 neural canal (which is formed by a dorsal invagination of the ecto- 

 derm). The neuroblasts develop secondary processes, many in the 

 neural canal, others after wandering from it, and by this process 

 of budding become nerve cells. The primitive processes continue 

 to extend until they reach the periphery of the body, are invested 

 with medullary sheaths supplied by connective tissue of the region 

 through which they pass, and become nerve fibers. Nerve fiber and 

 nerve cell are therefore parts of the same histological unit. 



The processes are of two kinds: i. («) unbranched; {}?) uniform 

 in diameter with lateral offshoots called collateral branches. These 

 are generally the central part of a nerve fiber and are called neurites 

 or axones. i. Processes which branch soon after leaving cell body 

 and break up into smaller branches called dendrites. Thus the 

 neuron consists of a cell body containing a conspicuous nucleus, 

 nucleolus and cytoplasmic granules, protoplasmic processes called 

 dendrites, an axis cylinder process which passes into a nerve fiber, 

 and a final termination in the form of a branching tuft. Golgi's 

 idea that protoplasmic processes or dendrites subserve nutrition of 

 the cell is without foundation. Many circumstances show that they 

 transmit impulses. 



The Sympathetic Nervous System consists of a pair of elongated 

 ganglionated cords extending from the base of the skull to the lumbo- 

 sacral region, connecting on the one hand by a series of branches to 

 the spinal nervous system, and on the other hand giving off an irregu- 

 lar series of branches to the viscera. At its cephalic end, each sym- 

 pathetic cord is continued in a plexiform manner into the cranial 

 relationships with certain cranial nerves. Caudally, the two cords 

 are joined by fine filaments and connected by the coccygeal ganglion. 



The sympathetic system rearranges and distributes fibers, derived 

 from the cerebro-spinal system, to the viscera and vessels of the 

 splanchnic area. It transmits afferent fibers from the viscera to the 

 cerebro-spinal system, and sends fibers to the vessels, involuntary 

 muscles and glands in the course of the somatic divisions of the 



