98 



CHORD ATE ANATOMY 



cases, as in the auditory organ and taste-buds (Figs. S4B, 348Z)), 

 the epithelial 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. 



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 anilin dye, methylen blue. These Nissl's bodies (Fig. 94) 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. 92), 



Bipolar cells. 



Fig. 93. — Diagram showing 

 how an embryonic bipolar nerve 

 cell is transformed into a unipolar 

 cell ("T-cell") such as occurs in 

 ganglia of the dersal roots of 

 spinal nerves. (From Bremer, 

 "Text-book of Histology.") 



Fig. 94. — Nerve cell, with processes 

 cut short; from human spinal cord. 

 X430. (From Bremer, "Text-book of 

 Histology.") 



extremely fine fibrils which are ordinarily 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 specialized avenues for conduction of 

 impulses. 



The neuraxon is a delicate thread consisting of a probably modified 

 protoplasm in which, as just mentioned, neurofibrils may be demonstrated. 

 It may be surrounded by one or two special ensheathing layers. The 

 medullary or myelin sheath is a relatively thick layer of fat-like substance, 

 myelin, fitting the neuraxon closely. The neurilemma or sheath of 

 Schwann is an exceedingly thin cellular layer wrapped around the neu- 

 raxon. (Figs. 91, 304) 



A neuraxon may possess either, both, or neither of these two sheaths. 

 When both are present the myelin sheath is always next the nerve fiber 

 and, at fairly regular intervals (in man averaging about 0.5 mm.) along 



