THE NEURONES. 



67 



Every neurone possesses at least one process, which is then an axone, 

 although usually provided with both dendrites and axone. Very rarely 

 more than one axone is present. Depending upon the number of their proc- 

 esses,' neurones are described 

 as unipolar, bipolar or multi- 

 polar. The unipolar neu- 

 rones occur only among the 

 lower vertebrates, the apparent 

 examples seen in the familiar 

 cells composing the spinal and 

 other ganglia connected with 

 sensory nerve-fibres resulting 

 from secondary changes. Pri- 

 marily, such neurones possess 

 an axone and a dendrite, which 

 pass from the opposite ends 

 of the young oval cell. Dur- 

 ing development, however, the 

 unilateral growth of the cell- 

 body brings about the gradual 

 approximation of the two proc- 

 esses until they fuse in the 

 single extension into which the 

 flask-shaped cell is prolonged. 

 Examples of bipolar 

 neurones, in which the den- 

 drite and axone pass from 

 opposite sides of the cell-body, 

 are found in the retina and the 

 ganglia connected with the 

 acoustic nerve.- An interesting modification of bipolar neurones is presented 

 by the olfactory cells, whose dendrites are represented by the short micro- 

 scopic processes embedded with- 

 in the nasal mucous membrane, 

 whilst the axones are prolonged 

 as the fibres of the olfactory 

 nerves. 



The cell-bodies of the multi- 

 polar neurones, which possess 

 one axone and several dendrites, 

 vary in form. Some, as those 

 within the sympathetic ganglia, 

 are approximately spherical and 

 of moderate size, with short 

 delicate dendrites; many are of 

 large size and irregularly stellate 

 form, the dendrites passing out 

 in all directions, as seen in the 

 conspicuous motor neurones with- 

 in the anterior cornua of the 

 spinal cord; others possess a regular and characteristic outline, as the 

 flask-shaped cells of Purkinje within the cerebellum or the pyramidal cells 

 of the cerebral cortex. Certain multipolar neurones within the cerebral 



FIG. 87. Nerve-cells of human spinal cord stained to show 

 Nissl bodies ; D, dendrites ; A, axones ; C, implantation cone ; 

 N, nucleus ; Af, nucleolus. X 400- 



II 



FIG. 88. Diagram showing 

 transformation of young bi- 

 polar sensory neurone into 

 one of unipolar type. 



FIG. 89 Bipolar neu- 

 rones ; a, from olfactory 

 mucous membrane the 

 dendrite is above; *, 

 from retina. (Cajal.) 



