MULTICELLULAR ANIMALS 



211 



the nerves that receive sensations in the human hand are located in 

 groups adjacent to the spinal cord, the fibres from these cells reach- 

 ing from approximately the middle of the back to the surface of 

 the hand. 



There are three types of neurons, distinguished by the number 

 and arrangement of the axones and dendrites (Fig. 140). The 



MULTIPOLAR BIPOLAR UNIPOLAR 



Fig. 140. — Monopolar, bipolar, and multipolar neurons. 



most primitive type, the multipolar neuron, may be compared 

 with the branching cells of the nerve net of ccelenterates, in that it 

 consists of a cell body with several fibres. The bipolar neuron has 

 but two fibres; the monopolar neuron has only one fibre leaving 

 the cell but that fibre is usually divided into two branches very near 

 to its origin. The nerve fibres of all types end in a more or less 

 numerous set of branches. In the case of nerves that receive stimuli, 

 or, the sensory nerves, each branch of the dendrite, the afferent 

 fibre, is connected with some sort of receptor mechanism, for ex- 



