318 



Basic Structure of Vertebrates 



Bipolar cells. 



Fig. 260 (Left). Diagram showing how an embryonic bipolar nerve cell is trans- 

 formed into a unipolar cell ("T-cell") such as occurs in ganglions of the dorsal 

 roots of spinal nerves. (Courtesy, Bremer: "Text-Book of Histology," Phila- 

 delphia, The Blakiston Company.; 



Fig. 261 (Right). Nerve-cell, with processes cut short; from human spinal 

 cord. (X 430.) (Courtesy, Bremer: "Text-Book of Histology," Philadelphia, The 

 Blakiston Company.) 



receptor cell itself produces a nerve-fiber which conducts to the central 

 organ — e.g., an olfactory cell and its fiber (Figs. 167, 249C). In such 

 cases one neuron serves as both receptor and conductor. In other cases, 

 as in the auditory organ and taste-buds (Figs. 167, 250), the epi- 

 thelial receptors do not produce nerve-fibers, but are intimately related 

 to the terminal twigs of afferent nerve-fibers whose cell-bodies lie in 

 some deep ganglion such as the acoustic ganglion or a spinal ganglion. 



Nerve cells vary greatly in size, but in general are relatively large. 

 They are often the largest cells in the body exclusive of eggs. Some 

 possess two or more nuclei, and the bulky mass of cytoplasm, invested 

 by a supporting membrane of connective tissue, may be penetrated by 

 narrow canals serving for an intracellular circulation of lymph. 



The most striking characteristic of the body of a neuron is the 

 presence of large masses of a granular substance which has a strong 

 affinity for the aniline dye, methylene blue. These Nissl's bodies 

 (Fig. 261) have been shown to become reduced in neurons which have 

 been excessively active, indicating that the bodies contain something 

 which is a source of energy for nervous activity. Less conspicuous are 

 the neurofibrils (Fig. 259), extremely fine fibrils which ordinarily are 

 seen only after use of special staining methods. Such neurofibrils may 

 form an elaborate system within the body of the neuron and may be 

 traced into the neuraxon and larger dendrites. The appearance and 

 arrangement of these neurofibrils strongly suggest that they are special- 

 ized avenues for conduction of impulses. 



The neuraxon is a delicate thread consisting of a probably modi- 

 fied protoplasm in which, as just mentioned, neurofibrils may be 

 demonstrated. It may attain extraordinary length, as when it extends 

 from a spinal ganglion of a giraffe to the extremity of a foreleg. It is 



