GUSTATORY AND OLFACTORY TISSUES 263 



among the heavy and far thicker supporting cells, which also have 

 much larger nuclei. In most vertebrates the gustatory cells are few 

 in number, and are collected into the central portion of the "bud," 

 while the supporting cells surround it. In this position the wider or 

 thicker middle portions of these cells, together with the narrow proximal 

 ends and the still more narrow distal ends, make the whole organ oval in 

 outline or melon-shaped. 



The ends of the supporting cells are not arranged evenly with the 

 surface of the surrounding epithelium, but form a depression or pit into 

 which the gustatory rods of the taste cells project. The nerve supply 

 comes as a series of afferent fibers from cells in or connected with the 

 brain centers. These fibers enter the taste bud and end in lateral con- 

 tact with the gustatory cells. The impressions received by the cells in 

 their contact with the food or other substances are transmitted as im- 

 pulses to these nerve ends which carry it to the nerve centers. In the 

 mammalian taste organs some of the nerve endings enter the epithelium 

 near the taste bud, but not in contact with the sense cells. While the real 

 sensory endings are thicker and of irregular outline, these outer ones are 

 thin and of smooth contour. They probably do not convey any gustatory 

 sensation. 



Another group of animals in which we can be almost certain the 

 sense of smell and taste are present are the insects. These creatures 

 have been proven by experiment to be able to smell much more keenly 

 than man, and perhaps are better able to smell delicate and diffuse odors 

 than the keen-scented mammals. A dog knows many individual persons 

 by their smell, but an ant is able to distinguish all the members of its 

 colony instantly by their smell and to make the power serve him almost 

 as usefully as sight serves man. The carrion insects, beetles, blowflies, 

 etc., can detect the presence of and find their way to any decaying pro- 

 toplasm as certainly and quickly as many other mammals find their way 

 to food by sight or a knowledge of its location. Among many forms 

 of insects (as also some mammals) the females in the breeding season 

 emit an odor that attracts the males from many miles away. This is 

 particularly true of some night Lepidoptera. The sense of taste has also 

 been proven by experiment to be present in many insect forms. 



We shall first study the olfactory tissues of insects. These organs 

 always are found on the antenna among certain possible tactile and other 

 sense organs. The real olfactory organ is a hypodermis cell which has 

 been differentiated into a nerve cell capable of being stimulated by 

 certain odors (substances in a state of fine division in the air and, possibly, 

 the water). This perceptory nerve cell has its proximal end (efferent 

 end) drawn out into a nerve fiber which passes into the body to enter 

 some ganglion. 



