24 THE FUNCTIONS OF ANIMALS. 



are stimulated by the excitation of the end-organ, and 

 conduct the stimulus inwards to what are called centres 

 or ganglia. 



In vertebrate animals the brain and spinal cord contain 

 a series of such centres, some of which serve for the 

 perception of the changes produced in the end-organs by 

 the stimulus, while others preside over the activities of the 

 muscles. As we ascend in the scale, we find that in 

 addition the brain possesses, to an increasing extent, the 

 power of correlating present and past experiences, and 

 of originating or inhibiting action in accordance with the 

 judgment formed. 



Thus nervous activities involve — (a) end-organs or sense 

 organs ; (b) centres or ganglia ; and (c) the conducting 

 nerves, some of which are afferent (or sensory) passing 

 from end-organs to ganglia, while others are efferent (or 

 motor) passing from centres to muscles. And in whatever 

 part there is activity, there is necessarily waste of complex 

 substances and some degree of exhaustion. 



It is interesting to notice, as a triumph of histological technique, 

 that Hodge, Gustav Mann, and others have succeeded in demonstrating 

 in nerve cells the structural results (cellular collapse, etc.) of fatigue, 

 and that in such diverse types as bee, frog, bird, and dog. 



Muscular activity. — The movements of a unicellular 

 animal are due to the contractility of the living matter, or 

 of special parts of the cell, such as cilia (see p. 102). In 

 sponges there are often specially contractile cells ; in most 

 higher animals such cells are aggregated to form the muscles, 

 on whose activity all movement depends. 



In many of the lower animals, e.g. sea-anemones and 

 sea-squirts, the contractile strands consist of long spindle- 

 shaped cells which appear almost homogeneous ; these are 

 called smooth muscle fibres. They occur in certain parts 

 of the body in higher vertebrates, e.g. on the wall of the 

 urinary bladder. A more specialised kind of muscle, pre- 

 vailing in active animals, consists of fibres which show 

 alternate light and dark cross bands; these are called 

 striped muscle fibres. The two kinds, unstriped and 

 striped, may be seen to pass into one another in the same 

 animal, and in a general way one may think of the former 

 as slowly contracting, the latter as rapidly contracting. 



