98 



THE NERVE TISSUE 



drites of neuron B, and passes into the cyton of this second nerve 

 celL From cyton B the impulse travels into the axon which extends 

 outward in the spinal nerve to muscle tissue and stimulates the 

 muscle to contract. Most reflex reactions involve more than two 

 neurons. This is especially true of higher mammals, where volition 

 exercises control over simpler reflex mechanisms. 



Dorsal septum of 

 spinal cord 



Ganglion cell 



ganglion 

 Sensory root 



Spinal nerve 

 Motor root 

 Ventral , fissure of Cyton of motor nerve cell ending in muscle 

 spinal cord 



Fig. 53. — Diagram illustrating a reflex arc. 



^Epiderm is 

 Derm is 



Muscle 



THE SYNAPSE. 



Neurons are associated with each other through the synapse, at 

 which point the terminals of the axon of one neuron come into 

 contact by contiguity with the dendrites or cell body of another 

 neuron. Such association may be effected by a basket work of 

 neurofibrillar processes from the terminal end of one neuron fitting 

 against the dendrites or cyton of the neuron receiving the impulse. 

 The synaptic type of association is accepted as the usual one, but 

 there is evidence showing that nerve fibers may be continuous at 

 these points, especially among the lower vertebrates. 



TYPES OF NEURONS. 



Neurons located in different regions of the nervous system have 

 definite functional demands made upon them, and associated with 

 their functioning is a certain arrangement of their processes so that 

 several types of cells may be classified as follows: unipolar cells, 

 neurons with a single process which arises from one side; bi])olar 

 cells, in which a single axon and a single dendrite process project 

 from opposite ends of the neurons; multipolar cells, in which numer- 

 ous processes project from different regions of the cytons; and 

 ganglion cells, in which two ditfcrciit processes, axon and dendrite 

 arise from one side, a i)scu(l()-uiiip()lar condition. 



