NERVOUS TISSUE 143 



and both are in connection with the sympathetic plexus around neighboring arteries. 

 The otic ganglion receives fibers from a prolongation of the tympanic nerve, and it 

 sends branches to the parotid gland. The submaxillary ganglion is joined by the 

 chorda tympani and sends branches to the submaxillary and sublingual glands. 



The lower ganglia of the glossopharyngeal and vagus nerves the 

 petrosal and nodose ganglia differ from the other ganglia in the head by 

 being temporarily connected with rudimentary ectodermal sense organs. 

 Their contact with the ectoderm is transient, however, and their cells 

 are considered to have come down from the superior and jugular ganglia, 

 respectively. They are thus strikingly analogous to the ganglia of the 

 sympathetic trunk, and it may be considered that instead of being con- 

 nected with their nerves by rami, they have remained in the main stems. 

 Moreover the vagus nerves produce myenteric and submucous plexuses 

 in the oesophagus and stomach, which are quite like those of the sympa- 

 thetic system in the intestine, but the fibers pass from the nodose ganglion 

 to these plexuses without the interposition of a ganglion comparable with 

 the cceliac ganglion. In addition to sympathetic fibers, the vagus con- 

 tains many direct fibers, which probably come especially from the jugular 

 ganglion. At present, however, both the upper and lower ganglia are 

 described as similar in structure and as resembling the spinal ganglia. 

 The opinion here advanced, that the nodose and petrosal ganglia are 

 sympathetic, must therefore be regarded as tentative. 



STRUCTURE OF NERVOUS TISSUE. 



Owing to the extent of the ramifying processes characteristic of nerve 

 cells, it is rare that an entire cell, even a small one, is included within a 

 single section. A motor cell, such as sends its fibers from the cord to 

 distant muscles, has never been seen as a complete, isolated structure. 

 From what is known of its several parts, however, a diagram of such a 

 cell may be put together, as shown in Fig. 131. At the top of the figure 

 is the nucleated cell body, which in different nerve cells varies in diameter 

 from 4-150 /*. Frequently this nucleated portion is referred to as the nerve 

 cell in distinction from the processes which grow out from it. The proc- 

 esses include the relatively short and irregularly ramifying dendrites, 

 which convey impulses toward the cell body, and a single fiber, the 

 neuraxon, chemically and physically different from the others, which 

 conveys impulses away from the cell body. If the various processes radiate 

 from the cell body in several directions, as in Fig. 131, the cell is described 

 as multipolar; if the neuraxon is at one end of the cell and a single dendrite 

 at the other, the cell is bipolar (Fig. 126); sometimes the nerve cell has 

 only one process and is unipolar, as in the mature cells of the spinal gang- 

 lion which have a T-shaped process, and in other cells in which dendrites 



