SUBPHYLUM VERTEBRATA 



335 



control and in the directing of the activities of the alimentary canal. 

 The organs of equilibrium are an example of internal receptors and will be 

 considered in connection with the ear. 



Frontal sinus 



nafe bone 



0/fcfC+ory 



cell 



Ethmoida/ 

 cells 



Olfactory 

 membrane 



Palate 



Opening of 

 eustachian 

 tube 



Posterior 

 nares 



Nerve 

 fibers 



Palate Nasal 

 septum 



B 



Supporting cell 



Anterior 

 pares 



Middle 

 and lower 

 turbinates A 



Fig. 232. — ^Olfactory organ in man. A, view of side wall of the nasal cavity to show 

 distribution of olfactory epithelium. Probes are passed through the passages leading to 

 the frontal sinus, the sphenoidal sinus, and the antrum in the cheek bone, all of which may 

 be infected from the nose. Olfactory membrane stippled. B, vertical section, made on 

 line ab in Fig. A, showing the nasal cavity of one side, with the nasal septum. A probe 

 is passed through the passage to the antrum. Olfactory membrane black. C, section of 

 a portion of the olfactory mucous membrane; highly magnified. 



A variety of tactile organs is found in various vertebrates and on 

 different parts of the body, but they share a common plan. Nerve end- 

 ings occur between epitheUal cells, and the impulses are produced by the 

 mechanical stimulation due to pressure upon these delicate endings. 



The lateral line organs, which are found along 

 the sides of the bodies and about the heads of 

 some aquatic vertebrates, have for a long time 

 puzzled zoologists and have had attributed to 

 them various functions. Recent investigations 

 by Parker, however, seem to show that these are 

 stimulated by vibrations in the water of too 

 great wave length and too little frequency to 

 cause sensations of sound and that they give to 

 the animal information concerning movements 

 in the water which are important in the securing 

 of food, the avoidance of enemies, and adjustment 

 to currents. 



The most important organs of smell (Fig. 

 232) consist of sheets of sensory epithelial cells situated in the nasal pas- 

 sages. Substances carried through the air in the form of fine particles 

 fall upon this sensory epithelium and are dissolved in the fluid secreted 

 OD its surface. The cells are stimulated chemically, and the result is the 



Surface of 

 tonorue 



Gustatory 

 cells 



fibers 



Fig. 233. — -Human taste 

 bud, somewhat diagram- 

 matic. Highly magnified. 



