572 



A TEXTBOOK OF PHYSIOLOGY 



spherical cell found in the spinal ganglia of mammals. The two pro- 

 cesses of the bipolar cell gradually approach each other during develop- 

 ment until they combine to form a T-shaped junction. 



In the cortex of the great brain are other characteristic neurons; 

 the cells are pyramidal in shape; branching dendrons arise from each 

 angle of the pyramid, and an axon from the middle of the base 

 (Fig. 310). 



In the cerebellum are found neurons with large, pear-shaped cells, 

 with a single axon at the base, and a wonderfully branched den- 

 dritic process at the stalk end. Throughout the grey matter of the 

 brain and spinal cord there are small association cells, known as the 

 Golgi cells, characterized by the fact that their axons, after a short 

 course, divide into many terminal branches. 



FIG. 312. Two VIEWS, TAKEN AT TWENTY-FIVE MINUTES' INTEKVAL, OF THE SAME 

 NERVE-FIBRE GROWING FROM A GROUP OF EMBRYONIC SPINAL CORD CELLS INTO 

 THE LYMPH. (Ross Harrison.) 



Microscopical preparations of the central nervous system are iden- 

 tified largely by the type and arrangement of nerve cells. 



The neurons are held together by a supporting tissue, known as 

 neuroglia. They are brought into relationship with one another 

 through their end terminations the dendrons and the axon. These 

 processes do not fuse together; they are in contiguity, not in con- 

 tinuity. The intertwining branches form a synapse; one neuron, 

 conducting an impulse, induces an impulse in another through the 

 synapse. The synapses only allow conduction in a forward, not in 

 a backward, direction. For example, a stimulus can pass up a motor 

 nerve-fibre as far as the anterior horn cell but not bevond it into 



