172 ANIMALS 



and those Reptiles and Amphibia which have an ear drum the 

 bones of the middle ear consist of one piece only, the columella. 



404. Caution. Our knowledge of the senses of animals is still 

 far from complete. Since we cannot experience the sensations 

 of another human being we can only, in a general way, infer what 

 they are by supposing them to resemble our own. Such an 

 assumption, with regard to the lower animals, is of little value. 

 We have senses of which we are unconscious (the sense of the 

 organ of equilibration and the senses of the deep lying organs 

 mentioned above), and the lower animals may have senses 

 which we do not have. The lateral line of Fishes, for example, 

 is a system of sense organs by which the animal is informed of 

 movements of the water. Many other sense organs have been 

 found whose function is unknown. 



405. Function of the Senses. Concerning sense organs in 

 general, it may be said that animals are provided with sense 

 organs for perceiving those changes in the environment which 

 might operate either to the advantage or detriment of the or- 

 ganism, and to which the organism is capable of making an ef- 

 fective response. Within the meaning of the term as used here, 

 we are insensible to those constant elements- of the normal at- 

 mosphere, oxygen, carbon-dioxide .and nitrogen, although 

 oxygen is absolutely and constantly necessary to life, while 

 carbon-dioxide in large quantities is fatal. Nor do we possess 

 organs for dectecting changes in the force of gravity, of at- 

 mospheric pressure or of electrical conditions, for the evident 

 reason that either the welfare of the organism is not affected 

 by the changes which normally occur or else that no effective 

 response is possible. 



ORGANS OF RESPONSE 



406. When an animal is sufficiently stimulated a response 

 occurs. This is usually in the form of a contraction or expan- 



