56 



CLASSIFICATION 



smell, and are acted upon by gases and vapours. Fragrance and 

 its contrary are the only smells to which the name can strictly be 

 applied, as some so-called "odours", such as those of a pungent 

 nature, are due to the delicate sense of touch connected with 

 the nasal cavities. The first cranial or olfactory nerves are those 

 of smell proper, and they supply the end-organs alluded to. As 

 already mentioned, certain sensations which we are accustomed 

 to call tastes are partly of the nature of smells. 



The Organs of Hearing (fig. 32). — These complicated struc- 

 tures consist of two subdivisions — (i) the essential parts 



containing the end-organs, 

 and (2) certain sound-con- 

 ducting arrangements, (i) 

 The essential parts con- 

 stituting what is known as 

 the internal ear are im- 

 bedded in the firm side- 

 wall of the skull, and con- 

 sist of a complicated bag, 

 the membranous labyrinth, 

 on each side. This bag is 

 really a small portion of the 

 skin, which has been folded 

 in so as to reach a sheltered 

 position and escape injury. 

 If the head of an embryo 

 chick be examined, a small 

 pit will be seen on either 

 side of it. This is the 

 commencing internal ear, 

 and later on the mouth 

 of the pit closes, converting it into a minute vesicle lying just 

 below the surface. This vesicle sinks inwards, and gradually 

 assumes the complex shape characteristic of the labyrinth in the 

 adult, while at the same time bony matter is formed round it. 

 The lining of the labyrinth is made up in several places of slender 

 auditory cells, which are the end-organs for hearing, and are sup- 

 plied by the eighth or auditory cranial nerves. (2) The end-organs 

 of hearing are stimulated by those vibrations of the air which are 

 known as sound-waves, but, owing to the deep position of the 



Fig. 32. — Diagrams of Auditory Organs {natural size] 

 Parts cut through are shaded, the close shading indicating the 

 temporal bone. Membranous labyrinth dotted. Ty, Tympanic 

 cavity ; viii, auditory nerve. Arrows indicate direction in which 

 sound-waves and resultant nerve impulses pass. A is after 

 Schwalbe, and B is a still simpler diagram. 



