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, 2.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 organtc 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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