CHAPTER LVIII 

 SENSE-ORGANS OF INVERTEBRATES AND VERTEBRATES 



Sense-organs are the intermediaries between the nervous 

 system and the environment, and essentially consist of ecto- 

 derm cells (end-organs) capable of being influenced by external 

 agents or stimuli. The stimulation of a sense-organ may be 

 immediately followed by a reflex action, or it may lead to a 

 voluntary action, and it is commonly associated, in higher animals 

 at least, with what is technically termed a sensation, i.e. an 

 awareness of something in the surroundings. Supposing that in 

 ourselves a light is suddenly flashed in the eyes when it is night. 

 The eye is first affected, then the optic nerve, and then some of 

 the nerve-cells in the brain. It is not till these last are brought 

 into operation that we " see a light ", and by comparison with 

 past experiences are put into possession of a piece of informa- 

 tion about our surroundings. It must be added, that besides 

 special sensations, such as those of hearing, sight, &c., there 

 are others of obscurer nature, which tell us something about the 

 state of the body itself, and are known as organic sensations. 

 Such are feelings of hunger, discomfort, &c., which, though of 

 great importance for the well-being of the body, since they often 

 guide to actions, e.g. feeding, which conduce to its welfare, will 

 not be considered here, since they are not related to special 

 sense-organs. Nor will reference be made to the " muscular 

 sense ", by which muscular efforts are gauged. 



It will be convenient to place the subject-matter of the 

 present chapter under the time-honoured headings of Touch, 

 Taste, Smell, Hearing, and Sight, for it is by means of sen- 

 sations which can be broadly classified in this way that we 

 derive most of our knowledge of surroundings. But many of 

 the lower animals possess sense-organs of which we can only 

 conjecture the use, and the stimulation of which must result in 



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