!88 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Therefore in the evolution of the organic kingdom 

 the most necessary and first evolved part of a hearing 

 organ was the vestibular sac and otoliths. These are 

 found in the higher invertebrates. Then in fishes there 

 were added the semicircular canals. Next the bone grew 

 about these parts, so as to shut them off from the brain 

 cavity, and at the same time inclosed them in such wise 

 as to form cavities of shape similar to the organs them- 

 selves. This is found in the amphibians. Then the mid- 

 ear was added, with its tympanic membrane and at least 

 one ossicle. This is the case in reptiles. Then in birds 

 a cochlea was added and a shallow meatus. Finally, in 

 mammals the cochlea was greatly elongated, and there- 

 fore coiled, more links were added to the chain of ossi- 

 cles, the whole apparatus was sunk deeper into the head 

 and with a longer meatus, and the conch was added. 



SECTION VIII. 

 Lower Senses. 



The three lower senses will be more rapidly dis- 

 patched, because the impression in these is directly on 

 the specialized nerve without the intermediation of an 

 instrument. We have therefore only the specialized 

 nerve and terminals to deal with, and the affections of 

 these are so inscrutable that we can have little to say. 



SENSE OF SMELL AND ITS ORGAN, THE NOSTRIL. 



The nostril in man is a quadrangular cavity, passing 

 directly backward from the face to the throat (Fig. 121). 

 It is covered in front by the overhanging nose, but in 



verse is true — i. e., the effect is greatest when the rods are struck 

 end on. The reason of the difference is that sound waves are 

 waves of longitudinal vibration, while light waves are waves of 

 transverse vibration. 



