298 



STUDIES IN PHYSIOLOGY 



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The necessary conditions for distinct sensations of smell 

 are these : (1) The odorous materials must be suspended in 



the air. Even when the nose 

 cavities are filled with liquid 

 cologne, we get no sensations 

 of smell. (2) The mucous 

 membrane of the nose must 

 be kept moist. In cases of 

 catarrh, when the amount of 

 mucus is lessened, the sense 

 of smell is either lost or im- 

 paired. (3) The olfactory 

 cells must not be stimulated 

 continuously. After one has 

 been in a close and even ill- 

 smelling room for a time, one 

 ceases to detect the odor. 



FIG. 134. Diagram of Lining of 



Nose. 



A = cells lying close together. 

 B = two cells separated. 

 c = cylindrical (supporting) cell. 

 n nucleus or nuclear region of 



cell. 



r = rod-shaped (sensory) cell. 

 8 = distal region of cells. 



5. THE SENSE OF SIGHT 



Protection for the Eye. 



The delicate organs of vision, 

 the eyes, are protected in a wonderful manner. In the first 

 place, the eyeballs are set far back in bony sockets, in such 

 a way that, even if one falls forward or if the head is struck 

 with a large object, there is little danger that the eyes 

 themselves will be hit. Again, each eyeball is covered by 

 two movable lids that involuntarily fly together when any 

 dangerous object approaches the head. And, finally, the 

 curving eyelashes on the edge of each lid protect the eye- 

 ball to a considerable extent from dust and dirt. 



The Tear Glands and Ducts. The exposed surface of the 

 eye is kept moist by the secretions of the tear glands. The 

 latter lie along the outer side of each eyeball, and are about 

 the size of an almond. From each gland the salty secretion 

 oozes out on the under surface of the upper eyelid, and as 



