202 THE HUMAN BODY 



fail, being unable to combine his muscles properly on account of 

 the conflicting afferent impulses. This and the feeling of rotation 

 in the contrary direction when a previous rotation ceases become 

 readily intelligible if we suppose feelings to be excited by relative 

 movements of the endolymph and the canals inclosing it. 



The sense of equilibrium as mediated by the semicircular canals 

 is a dynamic sense, one dealing with equilibrium of motion. That 

 we have also a static sense of equilibrium, which tells us our posi- 

 tion when at rest is well known. The swimmer immersed in 

 water knows perfectly whether he is on his face or on his back; 

 whether his head is up or down. This static equilibrium sense is 

 thought to be mediated by structures of the vestibule, the utricle 

 and saccule. These are hollow structures having stiff hairs pro- 

 jecting into their cavities and tiny stones caught among the hairs. 

 The weight of the stones will affect the hairs among which it rests 

 in one way when the head is erect, in quite another way when 

 the head is horizontal. Thus the nerves may be stimulated dif- 

 ferently for different positions of the head, fulfilling the conditions 

 that the sense requires. In many invertebrate animals structures 

 similar to the utricle and saccule represent their only organs re- 

 sembling our ears in any way. Experiments upon these animals 

 have shown that in them these structures are not hearing organs 

 but organs of equilibrium. 



Smell. The region of the nostril nearest its outer end possesses 

 the sense of touch : the olfactory organ proper consists of the upper 

 portions of the two nasal cavities, over which the endings of the 

 olfactory nerves are spread and where the mucous membrane has 

 a brownish-yellow color. This region (regio olfactoria) covers the 

 upper and lower turbinate bones, which are expansions of the 

 ethmoid on the outer wall of the nostril chamber, the opposite 

 part of the partition between the nares, and the part of the roof 

 of the nose separating it from the cranial cavity. The epithelium 

 covering the mucous membrane contains three varieties of cells 

 (2, Fig. 76) . The cells of one set are much like ordinary columnar 

 epithelium, but with long branched processes attached to their 

 deeper ends; mixed with these are peculiar cells, each of which 

 has a large nucleus surrounded by a little protoplasm; a slender 

 external process reaching to the surface; and a very slender deep 

 one. The latter cells have been supposed to be the proper olfac- 



