50 



PSYCHOBIOLOGY 



one axon may be in contact with the dendrites or the cell-bodies of a num- 

 ber of other neurons, and conversely, the branches of a dendrite may be in 

 contact with axon branches of several cells. Moreover, as many cells 

 have many dendries each, the multiplicity of possible connection from cell 

 to cell is very important. 



Although an axon cannot conduct a stimulus to a dendrite of the same 

 cell, stimulation may be received by one branch of an axon and trans- 

 mitted to other branches of the same axon. Thus, if a single muscle fiber 

 is caused to contract it may irritate the axon branch applied to it, and cause 

 the contraction of another fiber supplied by another branch of the same 

 axon. 



THE STRUCTURE AND INVESTITURE OF NERVE FIBERS AND NERVES. 



The fibers (axons or dendrites) consist of longitudinal fibrils em- 

 bedded in a protoplasm which is called neuroplasm. These fibrils seem 

 to run through the cell-bodies, and thus the fibrils in axon and dendrites 



Pat cells. rrSsE^K^SJ-S 



Artery, 



Bundles of nerve fibers. 



Epineurium. 



Perineurium. 



Endoneurium. 



Fig. 36. Cross-section of a portion of a human nerve. Magnified about 20 diam- 

 eters. ( Lewis and Stohr, Histology.) Seven funiculi are shown : these are composed 

 of bundles of medullated nerve fibers with irregular septa of endoneurium and are 

 wrapped in lamellar fibrous perineurium. The areolar epineureum which binds the 

 funiculi together contains many fat cells. 



appear continuous. This is not certainly established. All nerve fibers 

 which pass beyond the immediate vicinity of their cell-bodies seem to be 

 provided with one or more coverings. Three types of coverings have been 

 distinguished, which in the order from inner to outer, are: 1. A coat of 

 delicate cells whose origin is from the neural crest. This is the neurilemma, 

 or sheath of Schwann. 2. A connective tissue sheath: the sheath of 

 Henle. 3. A coating of myelin, a fat-like substance held in suspension 



