ANIMAL PHYSIOLOGY 69 



Light, for example, has always a definite influence on certain 

 simple animals compelling them to move in certain ways 

 and to continue moving until they have arranged their 

 bodies in a fixed position with regard to the direction of the 

 light rays. Certain chemical substances, as well as gravita- 

 tion, magnetism and other external agencies exert similarly 

 definite influences. These externally controlled movements 

 are called tropisms. Various other animal motions are of 

 such a definite character, always recurring in exactly the 

 same way under the same conditions of stimulation, that 

 they are called reflexes; and these also go to show, as do the 

 tropisms, that much of the behavior of the simpler animals, 

 and even more or less of that of the higher animals, is beyond 

 the control of the animal itself. • 



As we proceed upward in the animal scale we find a 

 gradual grouping into definite positions of a number of 

 cells that are specially sensitive to the different influences 

 acting on the organisms, and along with this definite groups 

 of muscular cells and definite nerve pathways for impulses 

 to pass from the sensitive to the motor cells, and more and 

 more complex connections of groups with groups. In the 

 highest organisms we have sense-organs which make us 

 exactly acquainted with the outside world; we have brain, 

 spinal cord, and nerves, which receive the impulses from 

 these and turn them through the muscles into all the motions 

 our bodies are capable of; besides we have all those wonder- 

 ful processes included under the names instinct, memory, 

 and reason. 



The special senses and their organs. — The organs of 

 sight, the eyes, are the only organs of special sense generally 

 conspicuous and unmistakably recognizable when present. 

 In the vertebrates the eyes, ears, nose, and taste organs are 

 always situated on the head, but in the invertebrates the 

 sense-organs corresponding to these are often scattered over 

 the body, and certain other organs are found which from 



