In the development of the metazoa, or multicellular animals, specialization has occurred, some parts of the body becoming muscles with the primitive motility much developed, some parts becoming digestive organs, some parts conductors (the nerves) and some parts becoming specialized receptors or sense organs. A sense organ is a portion of the body that has very high sensitivity to some particular kind of stimulus. One sense organ is highly sensitive to one stimulus, and another to another stimulus. The eye responds to very minute amounts of energy in the form of light, but not in other forms; the ear responds to very minute amounts of energy in the form of sound vibrations, the nose to very minute quantities of energy in certain chemical forms. There is only one thing that a sense organ always and necessarily contains, and that is the _termination of a sensory nerve_. Without that, the sense organ, being isolated, would have no effect on the brain or muscles or any other {189} part of the body, and would be entirely useless. The axons of the sensory nerve divide into fine branches in the sense organ, and thus are more easily aroused by the stimulus. Besides the sensory axons, two other things are often found in a sense organ--sometimes one of the two, sometimes the other and sometimes both. First, there are special sense cells in a few sense organs; and second, in most sense organs there is accessory apparatus which, without being itself sensitive, assists in bringing the stimulus to the sense cells or sensory nerve ends. [Illustration: Fig. 25.--Diagram of the taste end-organ. Within the "Taste bud" are seen two sense cells, and around the base of these cells are seen the terminations of two axons of the nerve of taste. (Figure text: surface of tongue, taste bud, pit)] _Sense cells_ are present only in the eye, ear, nose and mouth--always in very sheltered situations. The taste cells are located in little pits opening upon the surface of the tongue. In the sides of these pits can be found little flask-shaped chambers, each containing a number of taste cells. The taste cell has a slender prolongation that protrudes from the chamber into the pit; and it is this slender tip of the cell that is exposed to the chemical stimulus of the {190} tasting substance. The stimulus arouses the taste cell, and this in turn arouses the ending of the sensory axon that twines about the base of the cell at the back of the chamber. The taste cell, or its tip, is extra sensitive to chemical stimuli, and its activity, aroused by the chemical stimulus, in turn arouses the axon and so starts a nerve current to the brain stem and eventually to the cortex. [Illustration: Fig. 26.--The olfactory sense cells and their brain connections. (Figure text: axon to brain cortex, dendrites, synapses in brain stem, axons of sense cells sense cells in nose.)]