26 



MENTAL EVOLUTION IN ANIMALS. 



duced by irritants (chemical stimuli), effects of electrical 

 disturbance (electrical stimuli), or lastly, the passage of a 

 molecular disturbance from any other nerve-fibre with which 

 the one in question may be connected. 



Nerve-cells are usually found collected together in aggre- 

 gates, which are called ganglia, to and from which large 

 bundles of nerve-fibres come and go. These rope-like 

 clusters of nerve-fibres constitute the white threads and 

 strings which we recognize as nerves when we dissect an 

 animal. (See Fig. 3.) The relation of the clusters of 

 fibres to the cluster of cells is now such as to supply the 

 anatomical condition to the performance of a physiological 



FiG. 3.— Small Sympathetic Ganglion (human) with Multipolar 

 Cells. Magnified about 400 diameters. (Leydig.) 



process, which is termed Reflex Action. If we suppose the 

 left-hand bundle of fibres represented in the woodcut to be 

 prolonged and to terminate in a sensory surface, while 

 the other three bundles, when likewise prolonged, terminate 

 in a group of muscles, then a stimulus falling upon the 

 sensory surface would cause a molecular disturbance to travel 

 along the left-hand or in-going nerve to the ganglion; on 

 reaching the ganglion this disturbance would cause the 

 ganglion to discharge its influence into the right-hand or out- 

 going nerves, which would then conduct this disturbance into 

 the group of muscles and cause them to contract. This pro- 

 cess is called reflex action, because the original stimulus 

 falling upon the sensory surface does not pass in a direct line 



