GUSTATORY AND OLFACTORY TISSUES 259 



or in water. These impressions are such as distinguish the quality or 

 individuality of the substance, and only a certain proportion of known 

 substances can produce an impression, at least in the case of man. Many 

 different substances produce similar impressions, and would be identified 

 as the same by most animals. 



The power is one that we do not fully understand, partly because our 

 own organs of taste and smell are so poorly developed. The delicacy of 

 this perception in other animals is wonderful beyond belief, while in 

 still others it is very poorly developed or not at all. In the case of man 

 it is probably degenerate from a former condition of high efficiency. 



The cells that perform the function are of necessity epithelial in 

 character, and in the cases where we have seen them they are usually 

 very thin and elongate. There is no peculiar structure of their sensory 

 ends by which they can always be distinguished from some other per- 

 ceptory cells of the simpler types that we know certainly to be tactile or 

 other cells. In both we meet with a variety of rod- or hair-like perceptory 

 organs. In all cases the function must be determined either by expe- 

 rience, which we. can only do in man; or by homology, as is done in the 

 other classes of vertebrates ; or by experiment and observation, as must 

 be done in all other animals. Thus we cannot be sure of the exact 

 function of many of the organs that have been given the name of " ol- 

 factory " or " gustatory " organs in a large number of lower forms. 



In the vertebrates we find the gustatory and olfactory cells forming 

 two distinct types, the only strong bond between them being the fact that 

 they are both used to perceive chemical qualities. 



The olfactory cells are the more typical of the two and so similar in 

 all forms to those of the birds that we shall study them as found in the 

 common fowl, comparing them with the well-known form of man. The 

 epithelium on the olfactory prominence of the chicken (Fig. 228) consists 

 of the same three sorts of cells found in the mammals, a sustentacular 

 group, the more numerous in number, and a liberal number of olfactory 

 cells lying scattered among them, the whole resting on 'a layer of basal 

 cells. A basement membrane is so weakly developed as to be apparently 

 absent. 



The sustentacular or supporting cells have long bodies reaching from 

 the surface down to the peculiar layer of basal cells. Here the proximal 

 end of the cell branches once or twice, and its ends are attached to and 

 intermingled with the processes of the basal cells. The cell body is of 

 an irregular form, expanded to contain the nucleus somewhere in its 

 distal two thirds, and it does not show the strong granular differentiation 

 of proximal and distal cytoplasm that these cells do in the human tissues, 

 except that the distal end secretes mucus in small quantities, and in some 

 cases is apparently ciliated. False appearances of ciliation are to be 



