140 VKTERINAEY PATHOLOGY. 



well known fact that all living things (organisms) respond to 

 stimuli. A stimulus is that which excites or produces a tem- 

 porary increased vital action, or it is any substance or agent 

 capable of producing activity in a living tissue or producing 

 an impression upon a sensorv organ. The extent or degree 

 of response to a stimulus is directly proportional to the organi- 

 zation and complexity of the tissue and especially those tissues 

 which are: (a) capable of being stimulated; (b) capaljle of trans- 

 mitting an impulse; and (c) capable of interpreting the impres- 

 sions produced bv the impulse. The following discussion of 

 response to stimuli is confined to animal tissues because inflam- 

 mation aftects animals only. 



Protozoa, altliough of the simpliest structure, consisting of 

 a single cell, respond to the various stimuli. They respond to 

 light. Thus, if a portion of a cover glass preparation of living 

 amoel^ae be exposed to intense li.ght, the amoebae in the 

 lighted area will, in a short time, become restless and begin 

 to move about and will finallv move awav from the area of 

 light. Bv a speciallv arranged hot stage, so that there are areas 

 of different temperature, amoel^ae will be observed to ac- 

 cumulate in the areas of favorable temperature and emigrate 

 from those of unfavorable temperature. That is, they respond 

 to or are responsive to thermic stimidi. In a similar way 

 amoebae respond tn various chemical stimuli. If a drop of acid 

 be so placed that it will slowlv diffuse into the water or fluid 

 in which the amoebae are being studied, thev will move awav 

 from the acid. If an amoeba be dixided bv mechanical means 

 so that one segment contains tlie entire nucleus and the other 

 segment has no nucleus, it will l^e ol^served that the nucleated 

 segment responds to the stimulus bv regenerating tissus to re- 

 place the nonnucleatcd segment which was removed. On the 

 other hand, tlie nonnucleatcd segment of the amoeba mav 

 sur\-ive the shock of separation, Init soon begins to degenerate 

 and finallv dies. Thus is shown tlie response of living struc- 

 ture to photic, thermic, chemic and mechanic stimuli. 



If more complex animals ht considered there will be ob- 

 served a similar response to stimuli. Thus, the hydra responds 

 to the various kinds of stimuli and has a remarkable power of 

 regeneration of tissues. Vermes are very responsive to stimuli 

 and all observers have noticed that when an angle-worm is 

 cut in two both ends will crawl away. Vermes are among the 

 lo\\est forms of animals that possess cells corresponding to 

 white corpuscles or leucocytes of higher animals. These cells 

 are oliserved to emigrate to the point of injury or to surround 



