SENSE-ORGANS 133 



recognise the same mode of response of the animal's 

 nature to a stimulus that sets up new development. 



The Organs of Sense. — When this view of animal 

 life and development as an orderly response to inward 

 and outward stimuli has become familiar we are able 

 to consider the nature of the senses of animals. The 

 organs of sense are such as originate a message of a 

 definite kind destined for the nervous system. Besides 

 the five senses we have a muscular sense, one of heat, 

 of cold and of pain. In addition to these, messages 

 too faint for our brain to interpret are passing to it 

 from all parts of the body and informing it of their 

 several conditions ; to which messages it is able to 

 respond by assurances of which we are totally un- 

 conscious. The good results of rest, fresh air, change 

 of occupation, are the outcome of such toning effects 

 of the nervous system on the muscles and organs 

 generally, of whose needs we are only vaguely conscious, 

 but to which the nervous system responds with more 

 than a physician's skill and promptitude. 



In dealing with the senses, therefore, we shall 

 consider these subconscious senses, as well as those of 

 sight, hearing, and outer impulses. 



The psychical phenomena of animals present a 

 marked contrast with the physical. In structure 

 and mode of development animal life presents varia- 

 tions of endless diversity on several themes. The 

 complications that adapt creatures for life under 

 water, on land, and in air are of the most diverse kinds, 

 whereas the fundamental instincts possessed by this 



